Some basic info to get started. [TOC]
Books
Mathematics, Numerics, Derivations and OpenFOAM (Tobias Holzmann)
Introduction / overview of the underlying principles. Available for free. https://holzmann-cfd.com/community/publications
OpenFOAM Technology Primer (Mooney et al)
Introduction to programming with OpenFOAM https://www.researchgate.net/publication/267569764_The_OpenFOAM_Technology_Primer
Websites
TU Wien
Probably the best beginners tutorial available for free. Starting with a basic case setup and going to more complex cases. Very recommended to do in the beginning. https://www.cfd.at/tutorials
Holzmann-cfd
Website by Tobias Holzmann with a variety of tools, test cases and information. https://holzmann-cfd.com/
CFD-Online
The go-to-ressource for all questions OpenFOAM
https://www.cfd-online.com/Forums/openfoam/
For German speakers, cad.de has a Forum section as well (not as active as cfd-online, but sometimes useful as well)
https://ww3.cad.de/cgi-bin/ubb/forumdisplay.cgi?action=topics&forum=OpenFOAM&number=527
Chalmers
The infamous Chalmers page. Lots of short (~10 pages) documents on niche and not-so-niche OpenFOAM topics
http://www.tfd.chalmers.se/~hani/kurser/OS_CFD/
Wolf Dynamics
Lots of info on their website and youtube channel, also for more advanced topics
http://www.wolfdynamics.com/
https://www.youtube.com/playlist?list=PLoI86R1JVvv8JHTymlAmDChejiwm846Wl
OpenFoamBuilder
Small website which has some utilities to quickly create common OpenFOAM files.
http://openfoambuilder.org
The OpenFOAM groups are not very useful, but the following people are regularily posting interesting updates / news / tips
Tobias Holzmann (freelance + ESI Group) https://www.linkedin.com/in/tobias-holzmann-746348bb/
Holger Marschall (TU Darmstadt) https://www.linkedin.com/in/holger-marschall-62175683/thu
Nicolas Diego Badano (Consultant) https://www.linkedin.com/in/nicolas-diego-badano/
Inspiration for Git usage
Sometimes it might be useful to handle cases in git repos. Some inspiration on that: https://petebachant.me/track-changes-to-openfoam-case-files-with-git/