TITLE: Books of interesting mathematics aimed at mathematicians
QUESTION [15 upvotes]: I'm currently in my third year of an undergraduate degree, mainly doing courses in abstract algebra. I'm enjoying my degree, but every so often when I'm swamped with difficult proofs, I feel like I need some extra inspiration to give myself more mathematical energy and remind me why I got interested in maths in the first place - otherwise I wear myself out. I do things like watching videos on numberphile, but I feel like this is aimed at a much lower level than I'm at.
My question: What is a good resource that features advanced mathematics and isn't shy about going quite deep, but is still fairly recreational in its nature? I don't really care what form this resource takes, it could be blogs, books, videos etc.
Any help would be greatly appreciated.

REPLY [4 votes]: My personal favorite actual math book in the context of your question is Silverman and Tate "Rational Points on Elliptic Curves." 
Another excellent math-type book is Havil "Gamma Exploring Euler's Constant."
On a less technical level, here is a list from the Cambridge Math Dept. with an excellent selection:
http://www.maths.cam.ac.uk/undergrad/admissions/readinglist.pdf