I would have to say a lot, if not most has come from personal experience. That, and talking to others who have had success with species that we have been working with. Books have been good, but have played a very very minor role.
Books, lectures, experience, internet, communications with people who have experience with or knowledge of tarantulas. Not all sources are specifically about tarantulas - some sources deal with other arachnids, or even other sciences altogether - but information can be assembled from many sources. Direct observation and critical thinking contribute a lot.
First I used the search function on this site about 2 weeks before I became a member. I still use it. Watched a lot of youtube vids of responsible keepers. Then I picked up The Tarantula Keeper's Guide revised edition. Then I bought my first T.
A lot of what I learned was shutting up and listening (well... reading) what people had to say here. There is a lot of knowledge to be found on this board if people know how to find it (I.E. do an extensive search and then ask in the appropriate forum - not every question is scientific in nature).
i started out reading care sheets and forums until i realized how dangerous that was. then i started really searching for scientific papers. i can EASILY tell when someone has done no research into taxonomy based on how they identify stuff from pictures. if they continually claim rock solid IDs i know they haven't researched or retained single figure % of what they did research. there is nothing inherently bad about care sheets or forums... except 95+% of the info on them is, well, wrong. if you know who actually knows there stuff versus who is a parrot, a confabulator, or a run of the mill idiot then sheets and forums become a much better prospect for right learning... but you have to suffer through ALL the crap on them, too.
hobby books can be dangerous, too. the only book i really trust is the Schultzs' Tarantula Keeper's Guide cuz they are constantly checking it for completeness and accuracy and make revisions. and even there, there is some stuff i don't agree with 100%
actual science books are better... but a lot of times they don't have a lot of info that is obviously directly applicable to the hobby. and a lot read like greek unless you have learned the taxo and physio lingo before hand.
here's a clue... if your studying doesn't go through a period of needing to look up a word or term every ten minutes you aren't really biting into meaty enough material to get *good*
Mackenzie and books for the most part, and then doing extensive research on here for more specific questions...for example, when we were thinking of starting to breed our albos, I did a ton of researching and searching through old threads. I also did a ton of lurking on here in the beginning.