Pretty much since they were published, I've owned Games Workshop's "How to Paint Citadel Miniatures," "How to Paint Space Marines," and "How to Make Wargames Terrain." They seem to have served me well so far. A few days ago, however, I rounded out my collection of painting books with "How to Paint Citadel Tanks" and "IA: Model Masterclass Vol. 1" in the hopes that they'll give me some cool new techniques to use on my titan.
I have to say these two books have taught me so many new techniques to use for my larger models. I already knew the basics, but these have shown me how to weather my vehicles so they actually look like they've been used. I'm a little too far into my titan to be using stuff like that now, but I'm definately going to use some of these new weathering techniques around the barrels of the titan's weapons and things like that. Not only my titan, but all the future vehicles I make are going to look so much better because of these two books.
Additionally, the IA Masterclass book has a very detailed section in the beginning explaining what products they use and where to get them. Instead of using the blanket statement about how they only use GW products (the other painting books are guilty of that) the Forge World book explains what products they use to finish and convert their models. They even say where to get plasticard! I've always had such a hard time looking for that stuff and they've answered all my questions.
So if you've ever needed help with your vehicles and larger models, turn to "How to Paint Citadel Tanks" and "IA Model Masterclass Vol. 1." They'll be a big help.