All these books are free to read: either they're distributed with a free license, or they're in the public domain.
These books are available in MOBI format - good for Kindle.
Trevor Garrison Smith
Maria Lucia Ferruzza
Rajiv S. Jhangiani and Robert Biswas-Diener, editors
Leigh Dodds and Ian Davis
Richard Harding and Agustín Guimerá, editors
Google Developer Training team
Creative Commons Aotearoa New Zealand