If they do have access to either of these, then they can still only learn spells written in that spell book, or known by that mentor. No spell book will have more than a dozen spells, and most will have less than 8. Mentors may be better, but are harder to keep and may not be allowed by the Gamemaster. In addition just because a player has access to a dozen different forms of attack doesn't mean he will use them with any better results. Most spell casters will rely on a couple of spells for most of their needs, using the rest only in special situations.
Characters purchase more spells the same way they purchase or improve powers and skill, using CPs.
A GM may restrict what new spells can be learned though. If a character does not have this skill, or wants to avoid making the roll, they may learn a spell from a spell book that they have regular access to - or may be taught by someone else who knows the spell, such as a mentor or another character. Characters may only attempt to learn one new spell per gaming session. Spells Below is one sample spell book, including history, and contents. Gamemasters are not obligated to use spell books in their campaigns, but they can be a useful plot device.