- Official Manual (Wikibook) https://en.wikibooks.org/wiki/OpenSCAD_User_Manual
https://github.com/nophead/NopSCADlib
Contains loads of parts and is updated regularly. There are parts that can be directly used in constructions, such as extrusions or enclosures. The library also features parts such as steppers or ball bearings that are nice for constructions in which such prexisting elements are embedded.
https://github.com/adrianschlatter/threadlib
Allows to create all sorts of standard threads. I've already successfully used this for ISO M8 threads as well as R1/2 threads.
Actually not OpenSCAD, but just as simple to use. This is a Python library for CSG that uses signed distance functions (SDF) and is really really easy to get started with, since the API is super well designed. Fun fact: Use this if you don't have access to an Xserver (e.g. because you're trapped inside a Docker container and don't want to set up Xorg). Unlike OpenSCAD and its bindings, it does not require an Xserver to run.