''Language Visible'' is based on a 26-week series Sacks wrote for The Ottawa Citizen in 2000, covering one letter a week. But his passion for letters originated while he was writing his first book, the Encyclopedia of the Ancient Greek World, which included a section on the origin of the Greek alphabet.

In addition to tracing the evolution of the sound and shape assigned to each letter, Sacks answers teasing questions: Why is A -- ''like the fair-haired son of a wealthy family'' -- almost always better than B? Why is K nearly as erotic a letter as X and O are? How did the lower case i acquire her distinctive dot? (Since we have learned in chapter J that the letter I was J's ''mother,'' it is clear why I is a ''she'' and why the lower case j also has a dot.)

Sacks's quirky style makes the book as fun to read as it is enlightening. For instance, he notes that C is a pesky letter, as any parent of a toddler knows. Besides the ''k'' sound of cat, there's the ''s'' sound of cereal. That alone is confusing to my 3-year-old. But Sacks reminds me that C is far more complicated than that. There are a number of different-sounding ''ch'' combinations: church, choir. The ''sh'' sound has various spellings: champagne, ocean. There's a silent c: indict. The soft ''ch'' sound of chump being the only unique sound of C, the letter could easily be replaced by S, K or Q. Sacks writes, therefore, that C occupies ''a big-title job of patchy duties, jealously on guard against her colleagues. Sure, she still looks great, especially when she wears serifs. . . . But does that justify her salary?''

There is an occasional groan. Illustrating the similar sounds of ''brothers'' T and D, Sacks writes, ''we all know what happened when the cow tried to jump over the barbed-wire fence: udder disaster.'' And while the writing is often clever, Sacks ends some chapters with a clumsy tic. The end of Chapter L: ''And so farewell to L.'' Chapter O: ''Let's now say OOO and go.'' (The O's are hugs.)

Charts showing each letter's evolution are fascinating, but the book has design flaws: a map of the route letters took across languages, from Greek to Etruscan to Latin, in only a century might be visually compelling, but it is hard to know for sure, because no magnifying glass comes with the book. Also, captions are too small and sidebar typefaces are oddly inconsistent. Such poor designs are unfortunate in a book that begs for an inviting visual presentation.