I've gotten a couple of emails with the question "how do i add more spells to a spellbook" so I thought I'd make a beta-post.
First, you need to create a spellbook "'Manage spell books" under your profile. I doesnt matter if you have 1 or 24 books, you need to activate one before you can begin (see image). An active book has the star next to it and the word "Yes". Then you can just browse around on the spell-page and select the spells you want to either create a spell card from or use the button "Add selected to active spellbook". You can use the red (-) or the green (+) on the right side of every spell to toggle that spell (yeah, page reload i know..). And the final way is to visit a spell to read more about it. You can there toggle the spell in the active spellbook.