I saw this again today, but I know it happened earlier in L6 and I may even have posted this bug before - but can't locate it in the forum.
To find a book in the PB tool one needs to either scroll endlessly or use Ctrl-F find (thankful for that, don't fix this small nuisance by taking the Find away!). Find doesn't work by just identifying locations as in resources, but filters the list much like Inline Search does today (so for consistency one could replace it by a persistent search control that looks like Inline Search...). When one builds the book and focus leaves the tool, the Find box will sometimes disappear - but the list then will stay filtered:
Sorry, I currently have no list of steps to reproduce this consistently.
This is just a nuisance, since hitting Ctrl-F again will show the Find box with the applied filter:

and taking the filter away will allow to see all PBs in the tool again.