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. 

Nathan Jacobson's Basic Algebra I is pretty good, along with the sequel for the more brave of heart.
(Disclaimer I haven't read II, but I imagine it is also good).
