By this link you anybody can free download (or just read online) first couple chapters of “Fluid Power Basics” by Alan Hitchcox. This handbook is really just a basics and probably will be good enough for beginners. Unfortunately, full book is available for $59. But you always can read previous edition (from 2007) online by this link.
By clicking this link you can download very well illustrated eBook about pneumatic system design from Automation Direct. And it is really good book for beginners.
Here is a free eBook what I tried to use for proportional valve calculations. In the reality this book was not useful enough, but you can read it at least because it free for download. The book contains a lot of ads at the pages, so, probably, this is the reason why it free.
Very basics book for beginners from Yuken.