My impression is, and I'd really like someone with more experience to verify this, that you can get unlimited numbers of spells in every rarity level that you've opened with spell ranks. Let me give an example, say I have one White spell book. That entitles me to 3 common and 1 uncommon spell, but it doesn't limit me to that. Through trade or treasure hunting, I can acquire all the rest of the common and uncommon White spells. But I can never acquire rare or very rare White spells unless I get more White books.
Is that right?
It's a pretty strong argument to get 3 spell books in a bunch of different areas, isn't it?