Were the English language ever to need an official guardian, Professor David Crystal certainly looks the part. With a godly white beard, a pair of oval spectacles and the sort of suit that gave Open University its distinctive visual styling, he makes one feel secure in his judgments. And rightly so: Crystal, now an honorary professor of linguistics at the Bangor University, has been ploughing this academic field for 50 years.

But the professor would, I suspect, quickly shrug off such a custodial title – not out of modesty, but principle. Though many endangered languages need their champions, he would say, English does not require a guardian; it is vibrant and evolving and can fend for itself. The cacophony of conversation in the Charlotte Street Hotel bar where we meet seems to prove the point. Crystal's own voice, precise and steady and with a lilt that is suggestive of his youth in Liverpool, rises through it as though through a din of students waiting for a lecture.

Sitting with his wife, business partner and occasional co-author, Hilary, Crystal is here to talk about A Little Book of Language, the latest work of a prolific career. He already has more than 100 books to his name; some are academic but many are for the general inquisitive reader, including By Hook or by Crook: A Journey in Search of English and Shakespeare's Words, which was co-authored by his son, Ben.

For the Crystals, linguistics is clearly a family affair. In the jaunty early chapters of A Little Book of Language, Crystal notes how, when his four children were young, he would study them. "We're talking the 1960s, when the study of linguistics had hardly begun – people did not know, in a scientific way, how you developed language," he recalls. "Several of us linguists at that time would record our own kids, just to get some data. There was some literature on it then, but no day-by-day, blow-by-blow examples. I recorded all my children over the years in some shape or form. It's what linguists do. You don't talk to a linguist without having what you say taken down and used in evidence against you at some point in time."

Something must have rubbed off. Though his elder two children, Steven and Sue, eschewed academia, his daughter Lucy took up copywriting and his son Ben, an actor, is now following his father. "His book Shakespeare on Toast was a runaway hit – I wish I'd written it!" says Crystal, before rapidly, and self-effacingly, adding: "But I couldn't have – because it was so cool and modern and so street in its approach to Shakespeare. He has examples of hip-hop Shakespearians and I would never have dared put any of that stuff into one of my books."

A Little Book of Language is a simple history of all language, taking in phonetics, development, social uses, the internet, endangered languages and a touch of literature. In writing it, Crystal follows in the footsteps of the Viennese art historian Ernst Gombrich who, in 1935, wrote A Little History of the World. This slight volume was meant to be a simple retelling of the last two millennia for an audience of children, but its first edition was declared too pacifist by Fascist Germany and banned. Thirty years later, Gombrich reprised the project, translated the book into English and updated it, and it became a bestseller. Yale Press, noticing the success of such simplicity, has since developed a whole Little History series, and asked Crystal to write one on language.

This all sounds very innocent, but books for children can be a contentious issue – Gombrich's experience being an extreme example. Language, as much as history, is part of a national identity and cannot escape contemporary debates. And since Crystal began his academic career in the early 1960s, there have been dramatic shifts in how the English language is taught. "The ethos of 50 years ago was that there was one kind of English that was right and everything else was wrong; one kind of access that was right and everything else was inferior," he says. "Then nobody touched language for two generations. When it gradually came back in, we didn't want to go back to what we did in the 1950s. There's a new kind of ethos now."

What has replaced it is something far more fluid – descriptive rather than prescriptive, as the terminology goes. In schools, appropriateness has replaced the principle of correctness. "Now, one looks at all varieties of language and asks why they are used, says Crystal. "We are rearing a generation of kids who are more equitable and more understanding about the existence of language variety and why it is there."

This doesn't sit easy with the traditionalists, of whom there are still many – as Lynne Truss's bestselling Eats, Shoots and Leaves proved. That book was a dog-whistle call to all those who missed the old certainties of grammar textbooks. "It is interesting," notes Crystal, his usually cool delivery tinged suddenly with a hint of exasperation. "What did Lynne do after Eats, Shoots and Leaves? She wrote Talk to the Hand [a book about rudeness and courtesy]. Anyone interested in language ends up writing about the sociological issues around it."

Crystal calls this a "moral panic" over "mythologies" – his clearest example being the belief that text messaging is destroying children's ability to spell. "It's all nonsense, but people believe it."

He addressed this in his book Txtng: the Gr8 Db8, published three years ago, in which he found that "txt speak" accounted for

barely 10 per cent of the contents of the messages exchanged, and noted that abbreviations have always been part of the English language. Having solved that argument with some decent data, he tells me that he's now moving on to Twitter.

"On Twitter [which limits each written entry to 140 characters], you don't get the range of texting abbreviations you get in text messaging. It's a more sophisticated kind of communicative medium. You get semantic threads running through it. When you start counting thousands and thousands of messages, you suddenly realise that on the whole it's a new art form in the making."

The breadth of the internet means that language is morphing not just on grocers' signs and in school playgrounds, but on a far more fundamental level.

"All these different genres – instant messaging, blogging, chatrooms, virtual worlds – have evolved different sets of communicative strategies, which means that you can look at the language and say, 'That must be an example of a chatroom, that must be an example of a tweet,' and you can predict it."

In a curious counter-example, Crystal mentions that he had to speak to a Commons Select Committee last July on the propriety of language. It wasn't evolving English that was the problem. "There was a great deal of concern about what plain English was in a parliamentary context and how it might be improved, because there is a lot of unnecessary obfuscation." I wonder whether this was in response to the expenses scandal which broke in May, when some MPs claimed that they had been faithful to the wording of the rules. Crystal won't be drawn beyond one enigmatic thought: "It all comes down to language at the end of the day. This is what it says – but what does it say?"

Becoming involved in bigger arguments seems to be an occupational hazard for a linguist. Whether it be education, politics or neuroscience, we all have a vested interest in the implications of language. Our conversation turns to the recent news of a man who had been lying in a vegetative state for seven years before doctors managed to establish basic communication by scanning his brainwaves. "We are moving fast in a direction where you will be able to see what people are saying," says Crystal, optimistically. "We've got to the stage where you can see the complexity of language processing. We're not at the stage yet of being able to see clearly individual sentence patterns and words, but it's not long off."

Surely this has huge implications, not least for personal liberties? "It is the case that virtually every language issue resolves into a social or political or psychological issue," Crystal reminds me. "Language has no independent existence apart from the people who use it. It is not an end in itself; it is a means to an end of understanding who you are and what society is like. At which point, you know that a linguist has to bow out and say, 'This is bigger than me.'"

And again, Crystal, though eminently qualified, shrugs off any mantle of authority that I try to impose on him, and takes an insouciant sip of his mineral water.

The extract

A Little Book of Language, By David Crystal (Yale £14.99)