Listmania!
Mathematics textbooks.
Calculus: A Complete CourseCalculus: A Complete Course by Robert A. Adams
Buy used from: £25.99
I have seen the newer editions of Adams and I would wholly recommend that you buy the fourth edition.
Probability and Statistics (Pie)Probability and Statistics (Pie) by Morris H. DeGroot
Beautiful book on probability theory and use in statistics (though I understand it more from the probability perspective).
Groups and Symmetry (Undergraduate Texts in Mathematics)Groups and Symmetry (Undergraduate Texts in Mathematics) by Mark A. Armstrong
Buy new: £38.41 / Used from: £27.50
Pleasant group theory textbook covering basics of the theory concisely.and well.
Elementary Linear AlgebraElementary Linear Algebra by Stanley I. Grossman
Buy used from: £7.99
Grossman's elementary linear algebra book is the best one; it is quite a shock that amazon don't sell it.
Elementary Number Theory (Springer Undergraduate Mathematics) (Springer Undergraduate Mathematics Series)Elementary Number Theory (Springer Undergraduate Mathematics) (Springer Undergraduate Mathematics Series) by Gareth A. Jones
Buy new: £17.50 / Used from: £10.70
Good coverage of theory and many good exercises.
Elementary Differential Equations and Boundary Value ProblemsElementary Differential Equations and Boundary Value Problems by William E. Boyce
Buy used from: £19.67
This is a good book for a differential equations course in the first or second year of a maths degree for example. I personally have the sixth edition.