The links below are some particularly good online electronic books and educational videos. Given the standardized curriculum they are excellent resources even if they are not the recommended textbooks for your course and they are all free.
NOTE: the to optimize your budget if you are able to get the book list in time its worthwhile searching for your next years textbooks in April May June as this is when people will initially put them on sale. Don't rush, look for good deals, watch the stated condition, and don't be afraid to complain if the condition doesn't match the advert. Often times if there is a issue you get a refund and are told to give the book to charity. By choosing carefully you can often sell the books for the same as you buy them. HOWEVER be very careful of issue/year, and discuss with the teacher if its ok being one issue out - sometimes its fine sometimes not. Personally we use ebay a lot but also abebooks barnes&noble and amazon - always check.
You can also donate books to charity getting a tax write off, and several schools/students organize book sales where you get some money and some money goes to helping less well off schools.