Strictly power points when I use them at all. Enhancers are as stated (+X items add free spell "slots", while multipliers are on PP base).
No books required once a list is known.
Source (trainer, texts, magical place, ancient artifacts, whatever) required to learn a new spell list. Once the acquisition of knowledge has started, actual DP spending can occur "on the road" as needed (this is because my games use continuous timelines instead of distinct "adventure-downtime" format, which makes downtime really unpredictable).
I've also experimented with PP-less magic (basically, spellcasting result vs spell difficulty, modifiers and external conditions determine whether the spell cast is "free" or results in fatigue. Fatigue is cumulative and results in all activities being more difficult... including spell casting. Kind of a snowball effect but less predictable than PP spending and provides capability to cast low-level spells almost free in safe conditions. Tuning that is a pain and it obviously favours casters of all kinds, but my games are caster-heavy anyway (even NSU often learn spell lists, and most PCs are at least SSU), so it doesn't harm balance. However, you have to be able to cope with a team of fully buffed, invisible and flying characters when they have initiative (and the reverse, of course, when they don't). Another side effects is that combats are *fast*, even if mop-up might take time - full preparation makes the initial strike that much more impactful, and combat outcome is usually decided after a couple of rounds.