The study of Numerology requires more than simply reading a few recommended Numerology books. It also calls for many hours of practice, as well as reading books that may not appear to have much to do with Numerology, such as The Dancing Wu Li Masters (Gary Zukav), Gödel/Escher/Bach (Douglas Hofstadter), Beyond Numeracy (John Paulos), The Mystical Qabalah (Dion Fortune), Jung and Tarot (Sallie Nichols) and others. In order to become a good numerologist you need to have a solid knowledge base, but more importantly, you need a level of understanding that makes it possible for you to reach out and uplift and enrich the person who comes to you for advice. It is therefore your responsibility to first enrich yourself before offering your services to others.
As for recommended Numerology books, obviously, Numerology; Key To Your Inner Self: A Complete Guide to Understanding and Using Your Numbers of Destiny (Hans Decoz with Tom Monte) tops the list. Not only because I wrote it :-), but because it compensates for the most common shortcoming among other Numerology books: a consistent focus on the personalities of the numbers as they appear in different aspects of the charts. The most important and challenging facet of Numerology is a clear understanding of the role each single digit number plays. For that reason, we not only described the meaning of each number once or twice before moving on to the mechanics and other aspects of Numerology, but we also depict each number over and over again, from the angle specific to its location in the chart. A number found, for example, as a Life Path influences one very differently when found as a Heart's Desire. This helps a person become acquainted with the personalities of the numbers to the extent that he or she would be able to predict how a particular number will respond specific to the circumstances and position in which it is found.
The Numbers of Life (Kevin Quinn Avery) -- my favorite, but I'm not sure if it is still available
Numerology: The Complete Guide, volumes 1 and 2 (Matthew Goodwin) -- memorize, memorize, memorize
Cheiro's Book of Numbers (Cheiro) -- and other books by Cheiro ... getting serious now
The Romance in Your Name (Juno Jordan) -- trusty old favorite, anything you can find by Juno Jordan is worth reading
Your Days are Numbered (Florence Campbell)
The Numerology Kit (Carol Adrienne) -- quick little starter book ... fun, but lacking depth
Numerology: With Tantra, Ayurveda, and Astrology (Harish Johari) -- a different perspective
Numerology and The Divine Triangle (Faith Javanie and Dusty Bunker) -- unique, smart
Numerology; The Meaning of Numbers (Austin Coates) -- thinking outside the box
Numerology and Your Future (Dusty Bunker)
Numerology: The Power in Numbers (Ruth Drayer)
Numerology for the New Age (Lynn M. Buess)
The Secret Science of Numerology: The Hidden Meaning of Numbers and Letters (Shirley Blackwell Lawrence)
Numerology: Key to the Tarot (Sandor Konraad)
The Life You Were Born to Live: A Guide to Finding Your Life Purpose (Dan J. Millman) -- Pretty much everything about this book is wrong, including the calculation methods, however you will learn much if you just stick to reading the descriptions of the double-digit numbers
A final note. Generally, numerology books written before, say about 1985, tend to be more in-depth and based on the experience and practice of hundreds of years of research than those written in the last couple of decades. Numerology is not an exact science and is not regulated in any way. As a result, quite a few "modern" numerologists have taken neither the time nor the effort to gain a solid understanding of this science before hanging out a shingle.
While Numerology is a living science and the potential to discover new meanings and techniques is certainly possible, in order to do that, you first have to gain a knowledge-based foundation that helps you recognize and follow the underlying logic. Plug into that, and the reality that "everything is numbers" becomes second nature. You no longer practice Numerology, you live it.