The Buildroot user manual is where you want to start reading if you wish to understand how Buildroot works, or wish to change/extend/fix things. It is available as:
If you find any errors (factual, grammatical, whatever) please report them to us.
Free Electrons offers a complete 3-days training course on Buildroot. They also make the training materials freely available:
If you find that you need help with Buildroot, you can ask for help:
buildroot at buildroot.org#buildroot on Freenode