It sounds like you are in a similar position to some of our interns! We expect a lot out of them and challenge them as much as possible while integrating them directly into our engineering team. (But we also do ask them tough questions about rocket science before hiring, I don't want to make it seem like I am trying to make them struggle)
For rocket engines, there are a number of books that are useful:
- Rocket Propulsion Elements
- NASA-SP-125 (free)
- Liquid Rocket Thrust Chambers
For Pneumatics, reading some of the industry guides made by manufacturer's is an excellent start:
- https://trimantec.com/blogs/t/pneumatic-components-industrial-guide
- https://www.parker.com/literature/Pneumatics%20Division%20Europe/PDE-Documents/General%20Catalogues/PDE2600PNUK_Parker_Pneumatic_Catalogue.pdf
For Hydraulics, you can also check for industry resources made by manufacturers, or this excellent Coursera course on fluid power: https://www.coursera.org/learn/fluid-power
For pretty much everything, I would suggest starting with Wikipedia. It is a truly wonderful resource. I have been in this industry for 10 years and I am still learning things they didn't teach me in school on a weekly basis!