Q: where did my books go when i upgraded to yosemite

Not all books can be migrated, because the themes and book layouts changed.  These books will have been replaced by albums.

How Photos handles content and metadata from iPhoto and Aperture - Apple Support

If you need your books keep your iPhoto Library and update iPhoto to version 9.6.1, so you can still run iPhoto on Yosemite.  This version of iPhoto still works well.

See Barney's User Tip for details on how to update iPhoto for Yosemite:

  Can't Update iPhoto because it is not available in the App Store