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If you are looking for a treatment of a mathematical subject which is unconventional, then I'd recommend "Concrete Mathematics" by Graham, Knuth & Patashnik [ISBN-13: 978-0201558029] which is reasonably far-reaching, and perhaps "On Numbers and Games" by Conway [ISBN-13: 978-1568811277] which is specifically concerned with the theory of numbers and number-like entities. Both of these are very 'elementary', in the sense of operating from first principles.

If, on the other hand, you're looking for breadth-of-coverage, then consider the enormous dictionary "CRC Concise Encyclopedia of Mathematics" by Weisstein [ISBN-13: 978-1584883470] (also on-line http://mathworld.wolfram.com/; new shorter edition forthcoming), which is very good to "dip into", but would not really be suitable for cover-to-cover reading.

Another option, available both in paper and on-line is "NIST Handbook of Mathematical Functions" [ISBN-13: 978-0521192255] or "NIST Digital Library of Mathematical Functions" http://dlmf.nist.gov/. Obviously, this work is primarily concerned with various special functions, including those of trigonometry and combinatorics.

There are also various handbooks of mathematics, which arrange the material by topic, but with very little discussion. I do not have a strong view on which of these is best, as various options have their own merits, but perhaps that of Bronshtein & Semendyayev [ISBN-13: 978-3540621300] has relatively large breadth and contains much more explanatory text than is typical for a handbook.