Mobile Developer's Guide To The Galaxy: 16th edition

The independent handbook for beginners and experienced players in the app business, now in its 16th edition. More than 20 writers from the mobile community share their know-how in dealing with topics such as accessibility in mobile apps, UX design, mobile analytics, prototyping, cross-platform development, native development, mobile web and app marketing.
This is a non-profit project initiated by the German app agency Enough Software.
It is available for free as a pdf, epub or hardcopy at www.mobiledevelopersguide.com