[metasean]

A quick DDG [1] turned up this - http://hydraulicspneumatics.com/200/TechZone/PneumaticValves... which looks like the first chapter of an entire html book on the topic http://hydraulicspneumatics.com/ebooks/fluid-power-ebook-flu... (and the site also has a "Learning Resources" tab that looks like it will have even more for you when you're done with the ebook ;-) ).

[1] https://duckduckgo.com/?q=%28pneumatic+OR+air%29+%28logic+OR...