Sources of helpful information used in PS1 emulation(though some of it, especially older compiled information,
is wrong to some degree or incomplete, but it's still useful):

PCSX(and derivatives/forks)
MAME/MESS
P.E.Op.S

Blade Lib
psxsdk

doomed's PSX documents
bITmASTER's document
Neill Corlett's SPU documents
T.Fujita's SIO documents
ChrlyMac
Exophase
mizvekov
notaz
pSXAuthor
smf(blog)
shalma(forum posts, changelogs)
drhell's site
jac's CD-XA document
"The PlayStation 1 Video (STR) Format" - M. Sabin
Various PS1 emulator compatibility lists(for identifying problematic games)


MIPS RISC Architecture - Gerry Kane (1st and 2nd editions)
MIPS Programmer's Handbook, The

-----------
General(non-PS1-specific) CD/CDROM information and code:

SCSI-3 Multimedia Commands Revision 10A
ECMA-130

cdrdao
dvdisaster