Examples in Number Theory by Terence Tao [2005][PostScript]
math.ucla.eduSolving Mathematical Problems: A Personal Perspective would be more appealing. You can write a comment to highlight the part of the book that you like
Only first two chapters ("Strategies in problem solving" and "Examples in number theory") are available for free, so I took the second one as a title because it has some brilliant strategies.
On a whim, I tossed this at Google's Doc Viewer, to see whether that is the title -- and whether the Doc Viewer can now handle Postscript. Lo, it did, presenting me with a document titled problem.dvi . (Maybe Doc Viewer's done this for a while, but I remained ignorant.)