Book for algorithms beyond Cormen
As others have noted, books about (advanced) algorithms are best selected by topic. A good but heavy-weight general reference with rigorous analysis is probably The Art of Computer Programming by Knuth.

As for analysis techniques, you may be interested in An Introduction to the Analysis of Algorithms by Sedgewick and Flajolet, and Algorithmic Combinatorics by Flajolet and Sedgewick for more theory in the same direction.

For approaches on tackling hard problems, see Algorithmics for Hard Problems by Hromkovič.