My personal favorite to date are the
Tank Art books by Rinaldi Studio Press. I own Vol. 1 and have the Vol. 2 & 3 combo pack on pre-order (Vol. 2 is available, Vol. 3 will be released later this month). They offer incredibly detailed instruction on advanced weathering techniques including oils, powders, and enamels. The instructions are very clear and based upon the quality of the first volume, I cannot recommend them enough. They are also reasonably priced at about $30 each. The photos below are from the website:
The go-to standard for an advanced modeling bible is
FAQ2 by Mig Jimenez. It's about $80-$100 but is considerably longer than the Tank Art books. From the reviews I've read, it's mandatory reading if you're looking at weathering vehicles to the highest level. I don't have any personal experience with the book, but it's on my Christmas list along with a few other books available from AK Interactive.
I think you're on the right track by looking for books if you want to take your painting to the highest level. Youtube and the web are great places for tutorials, but there comes a point where you need to be willing to pay truly talented artists for their knowledge and experience. The trick is doing your research to figure out who is doing more than regurgitating the information you can legitimately get for free.
You also need to be honest with yourself about what you want to paint. As good as they may be, the books I've listed are useless if you want to be a figure painter or learn to master
OSL/
NMM. In addition, most advanced books will also require that you have an advanced toolkit and specific paints. For the books above, a quality airbrush and the skill to use it are prerequisites. Books are very cheap by comparison to all the other stuff you need.