I recommend you finish up your book projects in iPhoto before migrating over to a Photos library. As Larry said there are many differences in the themes, sizes and some themes are not included in Photos. In those cases your book will end up as just an album.
You can run both iPhoto and Photos at the same time on their respective libraries if you have iPhoto 9.6.1. If you have some version of iPhoto 9.X you can get iPhoto 9.6.1 after upgrading to El Capitan by following the directions posted by Tererence Devlin in this post:
Terence Devlin
Apr 14, 2015 11:21 AM
Re: Is Iphoto gone ? i want it back!
in response to Johannes666
Recommended
Go to the App Store and check out the Purchases List. If iPhoto is there then it will be v9.6.1
If iPhoto 9.6.1 is there, close the App Store, drag your existing iPhoto app (not the library, just the app) to the trash and then go back to the App Store.
Install the App from the App Store.
Sometimes iPhoto is not visible on the Purchases List. it may be hidden. See this article for details on how to unhide it.
http://support.apple.com/kb/HT4928
One question often asked: Will I lose my Photos if I reinstall?
iPhoto the application and the iPhoto Library are two different parts of the iPhoto programme. So, reinstalling the app should not affect the Library. BUT you should always have a back up before doing this kind of work. Always.
NOTE: the key to this procedure is to move the existing version of iPhoto 9 to the Trash bin in the Dock (don't empty yet) before going to the App Store.