are you sure you do not mix up the words? necromancia is for necromancers, and that is what your group seems to be- about dealing with the dead ones.
necronomicon is a book, but it is not related to necromancia at all. necronomicon is one more religious book, similar to our pagan traditions. it is said, it is ancient shumerian religion. you can get lovecraft's and simon's writings related to it. note this:
simon put a lot of interesting facts in his books, but it is hard to trace his identity what means he may spread propaganda etc. got no time to check on all the things he said, but many are true.
lovecraft was a journalist, seeking for sensations. reading his writings i tend to believe that often he writes supersitious's people's tales, mixing real facts with a hint of fantasy.