THE all-you-can-consume subscription often seems made for the Internet.

Netflix supplies a nearly endless amount of TV and movies; Spotify does the same for music. Pay a small monthly fee and then get as many episodes or songs as you can handle, for as long as your subscription lasts. It is a model that’s both convenient and enjoyable.

So why not add books to the subscription mix, too, right? That is the bet several companies are making, offering e-book subscription services that let you pay $10 a month or less for unlimited access to books in their catalog. The most noteworthy entrants so far are Oyster, Scribd and Amazon’s Kindle Unlimited, which became available in July.

The deal sounds appealing on its face. Someone who reads two or more books a month could save a little money. For those who read far more, the savings could be substantial.

But there is a hitch, and it is a big one: While the services each offer hundreds of thousands of books, many newer books are not yet available through these subscriptions. That is because the services haven’t been able to reach deals with many of the major publishers, especially for new books. So unless you’re a truly voracious reader who doesn’t mind older books, you probably want to avoid adding this monthly charge.