Lightroom 4 Book disappears, is erased or deleted.
Sharad, the product manager for Lightroom, has just posted a new blog entry on this book issue. It includes specifics on what can cause it and how to avoid it. The team has tracked down a reproducible set of steps that can be used to nail down a fix in a future update to Lightroom 4. Help with Disappearing Books in Lightroom 4