Just found this http://www.gtvault.com/gt4/tuning-guide/ . Most of it is pretty basic knowledge, but I think its a good read for novices and pro's alike.. It was obviously written for previous GT versions, but the info should still apply
GT Vault is the real deal; they've been around for quite some time now, and i've used alot of their tunes on my cars in GT4. There's a guy on there named fumes, he's the one of the best tuners out there.