The Best of Dover Books (a.k.a the best cheap mathematical texts) Perhaps this is a repeat question -- let me know if it is -- but I am interested in knowing the best of Dover mathematics books. The reason is because Dover books are very cheap and most other books are not: For example, while something like Needham's Visual Complex Analysis is a wonderful book, most copies of it are over $100.
In particular, I am interested in the best of both undergraduate and graduate-level Dover books.  As an example, I particularly loved the Dover books Calculus of Variations by Gelfand & Fomin and Differential Topology by Guillemin & Pollack. 
Thanks.
(P.S., I am sort of in an 'intuition-appreciation' kick in my mathematical studies (e.g., Needham))
EDIT: Thank you so far.  I'd just like to mention that the books need not be Dover, just excellent and affordable at the same time. 

In my opinion the best nooks are: 
"Ordinary Differential Equations" by Tenenbaum.
"Partial Differential Equations for Scientists and Engineers" BY Farlow
"Fourier Series and Orthogonal Functions" by Harry Davis
"Concepts of modern Mathematics" by Ian Stewart
There is a nice introduction to mathematics "Mathematics for the Nonmathematician" by Morris Kline.
You will find also lots of books on maths for fun like "The Moscow Puzzles" which is great. And don't forget the classic "Flatland" -it's just a story of two dimensional creatures-.
