TITLE: Insightful books about elementary mathematics
QUESTION [36 upvotes]: What are some books that discuss elementary mathematical topics ('school mathematics'), like arithmetic, basic non-abstract algebra, plane & solid geometry, trigonometry, etc, in an insightful way? I'm thinking of books like Klein's Elementary Mathematics from an Advanced Standpoint and the books of the Gelfand Correspondence School - school-level books with a university ethos.

REPLY [3 votes]: Two great ones are:

Fuchs, Tabachnikov: Mathematical Omnibus and  
Arnold: Lectures and    Problems: A Gift to Young Mathematicians.