How did you migrate your photos to Photos? You need to migrate the complete library to transfer the books to Photos.
The usual way to migrate your iPhoto Library to Photos is to open the iPhoto Library in Photos and let Photos convert it. To open an iPhoto Library in Photos drag the library onto the Photos icon or launch Photos while holding down the alt/options key and selecting the iPhoto Library.
That will migrate the books a well, as far as it is possible.
Not all books can be migrated, because the sizes and themes differ and will be migrated as albums.
See this Support page link:
How Photos handles content and metadata from iPhoto and Aperture - Apple Support
Some photo books and calendars, made with themes that are no longer supported in Photos, migrate to Albums.
To save your finished books it will be better to backup your iPhoto Libraries and to make sure, you update to iPhoto 9.6.1 if you have not done so already, so you still will be able to work with iPhoto books in there original version.
Only iPhoto 9.6.1 is compatible with MacOS X 10.10.3. Usually it will work to reinstall instead of updating:
See this User Tip:
Get iPhoto 9.6.1 if you didn't update before OS... | Apple Support Communities
The main idea is to move the iPhoto app to the Trash, so Spotlight does not see it as installed, reload the AppStore, and then to try to download again from the App Store - from your Purchases tab, not from the main page, since you can neither buy iPhoto nor update it, but you can reinstall, if your AppleID is associated with it.