Yes- the proper workshop manual!
The Haynes books can be a good start, but having the workshop manual to hand I've always found a bonus (when I've had access to it). They were about 50 squid at one point, but are now available on CD for significantly less cost (if you've the right vehicle!)
Out of the Haynes books themselves, the LR 90, 110 and Def book is most usefull, but less of a coffee table read! Again though, it depends if your year of vehicle is featured, so it's worth checking!
HTH
Cheers
8)
Eeyore