In my campaign, a standard spellbook is 100 pages, and each spell requires one page per spell-level* to inscribe. Therefore, the number of spells that can be written into a spellbook depends on the levels of the spells involved.
This file includes an array of check-boxes on the front page to indicate how many pages have been filled, and rows for the spell names, spell levels (and thus pages filled by them), casting/preparation time, and material components.
Using this form, it's a simple matter to keep track of which spell is in which book, how many books the character (or NPC) needs, and... and...
And probably some other stuff.
It should be handy for the DM too, for generating spellbooks-as-loot. The sheet can just be handed to the player when they manage to get past all the traps guarding the book. Or, if they don't... then not.
The PDF can be downloaded here. It's about 289KB.
* Note: I've converted all the spells into 20 spell levels, so for normal campaigns that would be more like two pages per spell level.