Two books with basic self training techniques are below. They get advanced in the later chapters and have focus on different topics, but the first 4-5 chapters of them all teach basic techniques that you need if you want to do anything related to the occult, such as relaxing your body, controlling your mind, and basic familiarity with working with energy. You need to be proficient with the stuff in the first few chapters of them before even thinking of touching demonic work.
Goetia & Pseudomonarchia Daemonum:
Companion Books to the Goetia. These are all part of the Lemegeton, which the Lesser Key is a part of:
Other good things to read if you are interested in demons: