Get a Bentley manual. https://www.amazon.ca/Volkswagen-Jetta-Golf-Service-Manual/dp/0837616786
Your first purchase should be this. I have an electronic copy if you want it. Do you know when the timing belt was changed last? If not, that's your second purchase.
There's a wealth of knowledge over at TDIclub.com, or just post up here. For the most part it's a pretty nice bunch.