I mostly use my hundreds of books, my declassified military reports, Mil-Specs, TO's, SACs, things like that.
Every once in a while, I go searching for an online source in order to be able to just post a link as an answer to someone's online question. That is just to save time from having to take a photo, post it, then host it, then link to it.
If I need something that I want to build, but, don't have a photo, I ask online, and then some expert will dig into his weapons manuals and give me his best ideas, sometimes with a good drawing of how it was done.
I mostly keep the two separate though, and use the internet for talking to other modelers, and books for references for my models.
Rex
(SWMBO's on-going Nag is that "you don't need a book for each model in your stash, you know!",,,but, she is behind, I have 550 unbuilt models,,,,and more books than models, lol)