We really like the check_mk documentation. It's comprehensive, verbose, and full of examples. Check some of the following links for more information:
- Quick manual installation guide: https://mathias-kettner.com/checkmk_turbostart.html
- Calling check_mk: https://mathias-kettner.com/checkmk_calling.html
- Catalog of check plugins: https://mathias-kettner.com/checkmk_check_catalogue.html