If I buy PS+, do I get every game in the free w/ PS+ Library, or do I only get access to the titles that become free after my purchase of PS+? There are tons of great games in the library, and I'd be mildly upset if I didn't have access to them.
you get the titles that are currently free
All games that are currently free you'll have immediate access to. All future games that will become free are also yours to download all depending on your status as an active Plus member. You can also purchase games at discount prices. Any games or other content that has a yellow Plus symbol next to it is currently attached to PlayStation Plus in some way or another, which means to say you'll be able to take advantage of those items.