The bigger hydraulics manufacturers offer in-house training and their books are available, sometimes for free from the agents.
To find a second hand book it is best to go through bookfinder.com which unites many bookseller search systems. When possible buy through the PF amazon link to donate a % of book price to PF.
Parker Fluidpower have released a series of more than a dozen training books on Hydraulics and Pneumatics.
Find them with bookfinder.com; author = 'Parker Hannefin', title = 'Bulletin'. Look for second hand copies of:
Author: Parker Hannifin Corp. Title: Industrial Hydraulic Technology. Bulletin 0232-B1. ISBN 1557690251.
Author: Parker Hannifin Corp. Title: Industrial Pneumatic Technology Bulletin 0275-B1. ISBN 1557690154
Also try bookfinder.com Title: 'Industrial Hydraulics Manual', and you will get Vickers, Eaton and others.
If you find .pdf copies of the Parker series of bulletins, or any other fluid training manuals, please post a link here.