Intermediate Text in Combinatorics? I'm currently attending a somewhat disorganized seminar on combinatorics that follows no textbook. So far we have covered the orbit-stabilizer theorem, some recursion, and we're heading into the Möbius inversion formula. 
Can anyone suggest a text that approaches combinatorics at this level for a 2nd-3rd year undergrad who already knows some algebra and the more basic combinatorics like combinations, permutations, stars-and-bars, generating functions? Most introductory combinatorics books I've found are more suited to a discrete math class and cover stuff which I already know. I'm looking for something to supplement this lecture. Thank you.
 A: I can't say I can predict what direction your seminar will head in, but an amazing one-stop-shop for combinatorics is Richard Stanley's Enumerative Combinatorics (both volumes). You will learn a tremendous amount if you get through both of these (and the revised version of volume 1 is currently available online!). In particular Volume I has a thorough discussion of Möbius inversion.
A: I did a reading course in Combinatorics while a PhD student, and we used van Lint and Wilson which I thought was very hard, but very good.  You don't need any more background than you already have, and you will learn a ton from this text.
It's definitely not as well-known as the other suggested texts, but it many ways it's superior.  (Although Concrete Mathematics is a better book overall for its sheer beauty.)
A: I think Stanley's Enumerative Combinatorics is not an "intermediate" level book, it's the canonical advanced book on combinatorics.
I'd recommend Aigner's  "A Course on Enumeration" (from Springer's GTM series)
for a lighter level (but certainly beyond apples and oranges) go A Walk Through Combinatorics (Miklos Bona) or Combinatorics: Topics, Techcniques and Algorithms (Cameron)
A: If you read French, you must read Analyse Combinatoire of L. Comtet at PUF edition. It's two short books full of fascinating materials.
A: How about Concrete Mathematics?
