Best advice I can give is to work from what you see first. Carry a sketch pad with you everywhere, and start drawing a wide variety of things. This develops two critical skills needed. The first is observation and REALLY taking a look at how things are connected and work together, whether its a human, a dog, a tree, or whatever. The second is hand-eye motor coordination, or the ability to transfer what you see to paper.
If you're looking for books, expand your search. Rather than focusing on Character Concept Design, look for anatomy, clothing design, weapons design, etc. Any elements central to the types of characters you want to design.
If you can provide more information on what type of characters you want to design, we might be able to provide more focused suggestions on reading material.
If you can already draw well, then taking this next step should come that much easier.
"If you love your job, you'll never work another day in your life."