Here I am collecting the books that I found interesting and informative. I am only listing those books that I've read and liked myself.
Numerology, the Complete Guide by Matthew Goodwin
This is a massive resource of numerological information. I found this book in a second hand book shop in London many years ago, and it opened my eyes on the rich possibilities offered by Numerology.
The book contains a very well structured, methodical and thorough description of many different numerological parameters, from the Life Path to the Maturity Number and the Secret Self. The book pays a lot of attention to synthesis - a logical interpretation of several different numbers.
If you want to have only one numerology book, I'd recommend this one. But make sure you get Volume 2 as well, it is devoted to numerological prognosis.