My degree is in 'Mathematics as a Double Subject', and I've taught Maths to A-level, but most books on mathematics have failed to really grab my attention. However Darrell Huff's book on the use of statistics is exceptionally well-written and likely to be of interest to anyone interested in mathematics:
http://www.penguin.co...atistics_Darrell_Huff
However the best book for the age group you refer to is long out of print. It's David Bergamini's book on Mathematics which was published back in 1963 (in the 'Life Science' series, from Time Life books). Secondhand copies are available here:
http://www.abebooks.c...&sts=t&tn=Mathematics
Chris