Hacking Tools and Referience Books

Tools:

-MIPS Assamber:
This tool can help you to convert N64, PSX and PS2 opcode to their hex representation.
MIPS Assambler

-Hex Convert:
With this you can convert decimal to hex base, also you can convert hex to float and vice-versa.
Hex Convert

-CD Mage:
A must have if you are into graphics modding, translations, allow to extract and reinsert files easily and automatically fixes TOC.
CDmage

-HxD:
Very powerful tool to Hex edit files, browse CDImages by sectors, comparing files, search for strings and much more good stuff.

Reference guides

-MIPS for PSX and N64 (English only)
This guide can help you to understand how MIPS works, also what are some of the opcodes the PSX uses. 
MIPS

-MIPS for PS2 (English only)
This guide can help you to understand how MIPS IV works and the new instruction set for the coprocessor unit in the PS2.
MIPS IV