The book that doesn't come with your motherboard!
Inside your computer are several smaller ones, which must work together for best performance. Getting them to do this, rather than pulling against each other, can make quite a difference to how your machine behaves, but most come out of their boxes inadequately set up because there isn't enough time or incentive for the manufacturers to do it for you - they mostly choose standard CMOS settings to suit the widest circumstances (and keep BIOS problems to a minimum), which are often the slowest. The BIOS Companion is a constant source of reference that explains those secret settings in the CMOS Setup, containing a wealth of data for troubleshooting, including Error and POST Codes.