Numismatic books perform a variety of functions: they authenticate; they differentiate; they historicize. Which is to say, they help determine whether or not a particular numismatic item is genuine or fake (counterfeit); they help determine in what way(s) a particular numismatic item is different from a similar numismatic item; and they place the production of particular numismatic items within a specific historical context. When such books are well illustrated and attractively printed and bound, they also stimulate one's aesthetic sensibilities.
Just as numismatic books perform a variety of functions, so do people collect numismatic books for a variety of reasons. At the risk of painting with an overly broad brush, numismatic books usually are collected for personal, professional or academic reasons (and sometimes for all three).
Example: you need a book to help you determine whether or not a numismatic item is real or fake. If you are a collector of this numismatic item, the appropriate book will help keep you from wasting your hard-earned currency. If a professional, this book will help keep you from sullying your reputation by selling something that is not genuine. If an academic, this book will help you explore the historical context in which counterfeits of the genuine item arose.
Among the numismatic books you may find in a private library are: bibliographies; dictionaries & encyclopedias; price guides; die studies; country-specific or denomination-specific studies; auction catalogs; numismatic journals.
Price guides are the part of numismatic literature with which the general public probably is most familiar:
As seen above, price guides are published for all three major numismatic specialties (coins, paper money, exonumia). For the most part, price guides assume that the numismatic item you have in hand is real. They merely provide some bare-bones information about it: who minted or printed the item; how many of this item were minted or printed over what time span; who did the art work (modeled the coin or engraved the banknote plate); what is the approximate value to other collectors of this particular item in a particular state of preservation.
That last bit of information is why most people buy numismatic price guides: to answer the question, "what is it worth?" As is true of books and other types of collectibles, the better the state of preservation, the more an item generally is worth (all other things being equal). For numismatic specialties where the value of an item is tied to its fabric (what the item is made of--e.g., gold, silver, bronze), the information contained in printed price guides may become obsolete too quickly. That is why publishers also make this information available electronically, through CDs & DVDs as well as via online access to real-time databases. Even with electronic access to such information, numismatic book collectors usually still purchase the print edition of such price guides for their private library (for historical data; in case electronic access becomes unavailable for whatever reason; because they do not buy & sell numismatic items that frequently; etc.).
Because price guides do not answer the question "is it real," numismatic book collectors also generally will have on their bookshelves several books designed to answer that specific question....