A really good way I found to introduce yourself to something is to go on Wiki and start reading a main article. Then start clicking linked words, even if you are somewhat familiar with it. This way you get an idea for the vocabulary of the subject you're looking into and related subjects. You can get a very basic understanding of a lot of things that way and then you have a good jumping-off point for further investigation.

Used book stores are wonderful for text books and stuff. The one near me has a free section for stuff they can't sell (older text books that have many new editions). And generally you should be able to find used versions of current books that are cheaper.

Then there's free ebooks. Google has an academic section. I think project Gutenberg has some science and math stuff.