Imaginator has a good point. There are some methods of going "outside the box" that could potentially help with your issue. I don't personally like the idea of ditching the Spellbook however, and I'm not certain if it would assist your problem of maintaining multiple spellbooks all that much. Another solution, however, is the
modular spellbook idea. First suggested by IndyPendant, it would still involve deconstructing books to create new ones when you wish to dabble with a new book, but it improves upon the concept greatly. Instead of swapping one spell at a time, you simply take a whole page of the book out and transfer it over. You can keep multiple half finished spellbooks around, and simply transfer the remaining shared spells as needed in a matter of seconds.