Artists manual to materials and techniques
There are so many artist's manuals, but this is one of the most extensive and broad I found. Books like this help me to improve my vocabulary of art and art techniques. Also, a manual always helps when you find limits with your intended media. It is never bad to have a good guide next to you while working on illustrations or doing paintings.
Understanding the materials you're using also improves your skills. I found this manual in our local library and loved to find answers to some questions I would have had a hard time to find the correct answers online.