One of my favourite texts is Types and Programming Languages (TAPL) by Pierce. This has a strong focus on types (as the name suggests), but basically is also a very in-depth (but readable) work on the semantics and implementation of functional languages. You will know more than you may ever want to about FP by the time you're done with it.
You may, or may not, have advanced beyond Purely Functional Data Structures already.
The state of textbooks and monographs on the subject of functional programming, and especially on how to design functional programs, is woefully inadequate. I see other answers recommending Pierce's Types and Programming Languages and Okasaki's Purely Functional Data Structures. Both are excellent books. Both are deep and narrow.
For a broader, shallower view that still "goes beyond the basics", the best source I know of is from a series of summer schools in "Advanced Functional Programming". These volumes are published by Springer. There have been at least six volumes; the most recent I know if is from 2008. I actually like the earlier volumes a bit better; the very first is from 1995. Follow either of these links and you will get to a Springer page; you can find the others by searching for "Advanced Functional Programming" on that page.
The primary advantage of this series is that each article is carefully crafted and is aimed roughly at beginning graduate students. So for a person who is comfortable and wants to go beyond the basics, the level is just about perfect.
The primary disadvantage is that both the individual volumes and the series lack the intellectual coherence that you would expect to find in a textbook or a monograph. It's simply a rich smorgasbord of cool ideas. Many of the articles are quite enjoyable, but you will build breadth, not depth.