What does "per book" mean?

A book is a single project in your library — one title, with its own manuscript, cover, and settings. When you unlock that project, the unlock covers:

• Every re-export of that book, forever. Revisions don't cost more.
• Every variant the tools produce for it — for the cover, that means up to 11 wraps (four printers, in every binding they sell at your trim) under the one license.
• The large print edition of it. See Is the large print edition included?

A different book — a separate title, a new project in your library — is its own purchase. If you're publishing three books, that's three books' worth of unlocks.

The gray area is "is my second edition a new book?" — see Can I use one license for a revised edition?