David Wells has assembled an unique and readable collection of facts about numbers, arranged in numerical order! Entries are fascinating, for the most part, though they can be frustrating, too. For example, when first encountering the notion of automorphic numbers (numbers whose squares end in the same digits as the original number), it is tempting to discover if there are other related entries -- 'trimorphic numbers', for instance? It is possible to track these down using the small index provided and quite fun to do.
Unlike other dictionaries this is best read from front to back though it can be used as a reference, once one is familiar with it.
Many concepts are briefly explained as they are encountered, and some merely referred to in passing, and the frustration here is that there need not be full information in the text. However, this is most enjoyably resolved by brushing up one's own skills and trying to demonstrate the simpler claims for oneself. There is plenty here for the dabbling amateur to try, though the serious mathematician can also enjoy the book.
I have one qualification: David Wells identifies 51 as the least uninteresting number (no, not a contradiction: it is simultaneously interesting and uninteresting, he claims) -- because he cannot find an interesting fact about it. He fails to notice that it is the fourth trimorphic (and non-automorphic) number: 4, 9, 49, 51 and 75 being the first five cases. This means that it is mildly more interesting than at first supposed.
I look forward to a revised edition -- with readers' contributions and comments.