Here's a list Oxford suggests. Not sure how much they are the sort of thing you're looking for.
Bridging Material
Allenby Numbers and Proofs (1997) 0340676531
Bostock, Chandler, Rourke, Further Pure Mathematics (1983) 0859501035
Liebeck, A Concise Introduction to Pure Mathematics (2000) 1584881933
Popular Mathematical Texts
Acheson 1089 and All That (2002) 0198516231
Brown Philosophy of Mathematics (1999) 0415122759
Clegg A Brief History of Infinity (2003) 1841196509
Courant, Robbins and Stewart What is Mathematics? (1996) 0195105192
Devlin The Millennium Problems (2004) 1862076863
Devlin Mathematics: The New Golden Age (1998) 0140258655
Dunham Journey through Genius (1991) 014014739X
Du Sautoy The Music of the Primes (2003) 1841155799
Gardiner The Colossal Book of Mathematics () 0393020231
Gowers Mathematics – A Very Short Introduction (2002) 0199504687
Gray The Hilbert Challenge (2000) 0198506511
Hilton, Holton, Pedersen, Mathematical Reflections (1998) 0387947001
Körner The Pleasures of Counting (1996) 0521568234
Polya How to Solve It (1945) 0140124993
Sewell (ed.) Mathematics Masterclasses:: Stretching the Imagination (1997) 019851493X
Singh Fermat’s Last Theorem (1998) 1857026691
Singh The Code Book (2000) 1857028899
Stewart, I. Concepts of Modern Mathematics (1995) 0486284247
History and Biography
Berlinski Infinite Ascent – A Short History of Mathematics (2005) 0297848518
Burton The History of Mathematics (2007) 0071106359
Goldstein Incompleteness – The Proof and Paradox of Kurt Gödel (2005) 0393051692
Hellman Great Feuds in Mathematics (2006) 0471648779
Hodges Alan Turing: The Enigma (1992) 0099116413
Hoffman The Man Who Loved Only Numbers (1999) 1857028295
Hollingdale Makers of Mathematics (1988) 0140149228