There are plenty of freediving books in several languages listed in the Freediving Media Base. You can purchase most of them even in a land-locked place. On the pop-up info card (shown after clicking a line in the media listing), there is mostly a BUY button at the bottom left (when vendor known) that redirects you to Amazon or another online merchant, where you can order the books:
General freediving books:
http://apnea.cz/books
Scientific books:
http://apnea.cz/medical+books
And there are even more scientific online documents (I mean other than paper books) listed in the database - some of the links show only abstracts, and you need to pay a small fee for the entire document, but a big number of the documents are available in full text online for free. There is enough reading for many months:
http://apnea.cz/medical+documents
I am permanently adding new interesting documents to the database, so you can check it often. You are also welcome to
add books, documents, or other media not yet listed on apnea.cz
