Archive.org and Google Books (limit search to full text books) have many, many more horse-related books than Project Gutenberg. And they are scanned versions, so you can see illustrations as they should relate to the texts. Plus they are starting to provide EPUB versions of the books for readers that can handle that format.
Archive.org has a number of PDFs from Google imported, so it's the best first place to search if you're going by title/author.
But Google can do full text searches on the body of the books, which comes in very handy sometimes.