Before getting into veterinary and mammalogy, you might want to start with the basics. As many people have mentioned, Campbell and Reece is one of the best textbooks to start a foundation in biology. If you live near a public library you might find a 6th or 7th edition to borrow. Amazon has a few 7th editions for cheap as well. Although it is an introductory biology book, it will give you a wonderful tour of cellular and molecular biology that will make it much easier when studying animal physiology (which they also covered in the later chapters to a limited extent).
Free Biology Text Books:
Boundless Biology: Boundless is a company that has many free textbooks for high school level biology to help teachers in have a free online textbook.
I have also found a list of free biology text books here that might be of use.
Free Mammology Textbook:
As part of a National Science Foundation Grant, the Northwest Center for Sustainable Resources created *Introduction to Mammology*. It is a free textbook that was developed for a two-year Wildlife Technology program. It also contains information related to the lab portion of the program, but you can skip those and focus on the material.
Free Online Coursework:
Coursera: A few people have already mentioned Coursera, which is a Massive Open Online Content (MOOC). Here, you can find many courses across a wide range of biology topics (and others!). You can enroll in a course (for free) and many times they come with accompanying free textbooks as well (at least that is what I have experienced).
edX: Another MOOC that was started by Harvard.
Khan Academy: Another great website for explanations of certain aspects of biology. As Tyto alba stated, there are some errors in the videos.