This book provides a nice introduction to probabilistic methods of proof and how they’re used in a combinatorial setting. Although some of the results may be dated now, there’s a wide range of topics covered, making it clear how we can use the powerful non-constructive method of proof. I would definitely recommend having an undergraduate level of understanding in both probability theory and combinatorics, the authors have an engaging and often humorous prose, making this a must-read book for the avid mathematician.
Long out of print, Erdős and Spencer's classic "blue book" introduced the elegantly powerful probabilistic method of construction, absolutely critical to analysis of randomized algorithms and a regular surprise guest in all manner of scattered proofs. I really ought go find an ebook of this and read it sometime; the book itself would be a nice find, but I don't expect to see it below $100 anytime soon, sigh.