Building a liquid-propellant rocket engine is pretty much the pinnacle of amateur engineering projects.
The 1967 book [How to Design, Build and Test Small Liquid-Fuel Rocket Engines](http://www.risacher.org/rocket/) can be read online, or you can download the entire file in [zip](/pdf/rocket.zip) format.
From the introduction:
>The purpose of this publication is to provide the serious amateur builder with design information, fabrication procedures, test equipment requirements, and safe operating procedures for small liquid-fuel rocket engines.
Also, don’t forget the bible:
[![Rocket Propulsion Elements](/images/rpe.jpg)]