Unconventional Mathematical Theorems Beyond Textbooks

[zonk]

I'm looking for something like a books filled with theorems not typically covered in textbooks. For example Viete's formulas for roots and coefficients of polynomials, which, strangely enough, I find useful. They speed up my computational speed and give interesting relationships between objects. A book that presents many interesting theorems that are not heavily utilized in standard education for different fields in mathematics. Anyone know a book like that? There are many interesting theorems on wikipedia that look interesting, but I don't care too much about the way they are presented.

 

[mathwonk]

as a student i liked counterexamples in analysis by gelbaum and olmstead