Move over, phones! Make room for books that fit in a back pocket.

Maggie Van Nortwick cradles the book in her palm and flips upward through its razor-thin pages, pausing now and then to read a paragraph or two. Finally, she looks up with a smile.

“This is cute,” she says. “But kind of a curveball for me.”

The book in question, John Green’s debut novel “Looking for Alaska,” measures no more than a few inches on each side. It is cellphone-sized and, unlike regular sized books, can fit easily into a back pocket or small purse. The type is small but readable; the pages barely opaque.

Ms. Van Nortwick, 18, likes the new format, but it may take some