This git-annex repository collates freely available online books on various sub-fields of Machine Learning. It doesn't physically contain any of them, but rather contains references to original websites, from which you could fetch them using git-annex. For instance
git clone git://github.com/datalad/mlbooks
cd mlbooks
git annex get *.pdf
would fetch all the books available as a .pdf file.
Originally inspired by 12 Best Free Ebooks for Machine Learning. Books themselves are available under varying terms. The collection itself is made available under PDDL.