I buy as many as I can on my kindle, as they're much cheaper on kindle! But not all of them are available on the kindle yet
I spent £70 on books the other week!!
My complete list of books is on a piece of paper, and I also have a list on my phone which I update every so often
Here is the list off my phone, I've read some of these already though:
Fermat's Last Theorem - Simon Singh.
A Brief History of Time - Stephen Hawking.
A Mathematician's Apology - G.H Hardy
QED: Strange Theory of Light and Matter - Richard Feynman.
God Created the Integers - Stephen Hawking
50 Physics ideas you really need to know - Joanna Baker.
Surely you're joking Mr Feynman! - Richard Feynman.
Six Easy Pieces - Richard Feynman.
Nothing: A Very Short Introduction - Frank Close.
Particle Physics: A Very Short Introduction - Frank Close.
Antimatter - Frank Close.
Dr.Riemann's Zeros - Karl Sabbagh
A Concise Introduction to Pure Mathematics - Martin Liebeck
Chaos: Making a New Science - James Gleick
What is Mathematics? - Richard Courant
The Pleasure of Counting - Thomas William Korner
The man who loved only numbers - Paul Hoffman
Finding Moonshine - Marcus du Sautoy
Mathematics: a very short introduction - Timothy gowers
Why E = MC^2 - Brian Cox
Alex's Adventures in numberland - Alex Bellos
Big Bang - Simon Singh
Makers of Mathematics - S.Hollingdale
Alan Turing, the Enigma - A.Hodges
The Man who knew infinity - R.Kanigel
Littlewoods miscellany
Number - J.Mcleish
Penrose Tile to Trapdoor Ciphers - M.Gardner
To infinity and beyond - Eli Maor
A Mathematical mosaic - Ravi Vakil
Solving mathematical problems - Terence Tao
From here to infinity - Ian Stewart
What's happening in the mathematical sciences - B.Cipra
Archimedes revenge - P.Hoffman
The mathematical experience - P.J.Davis
The penguin dictionary of curious and interesting numbers - D.Wells
New applications of mathematics - C.Bondi
Reaching for infinity - S.Gibilisco
The new scientist guide to chaos - N.Hall
Hidden unity in natures laws - John.C.Taylor
The cosmic onion - Frank Close
The quantum universe -T.Hey
Was Einstein right? -C.M.Will
The accidental universe - P.C.W Davies