What are some good condensed matter physics books that can fill the gap between Ashcroft & Mermin and research papers? Suggestions for any specialized topics (such as superconductivity, CFT, topological insulators) are welcomed.
To cover that gap you will have to study many-body physics.
Similar level than Ashcroft-Mervin (although modern and complete)
Many-Body Physics (General)
A good introduction, it covers lots of topics although notation is a bit old-fashioned. Some chapters are not very good (skip the quantum Hall effect chapter!).
Very good and cheap, specially if you want to learn Feynman diagrams applied to condensed matter physics problem.
A russian classic by one of the masters. Also a bit old fashioned and not very easy for beginners but covers all the basics.
Already mentioned in the other answer. For a path-integral approach to condensed matter physics.
Very well-written and easy reading. Similar to the first one (Mahan).
Quantum Hall Effects
I don't like it very much, very sloppy with notation.
The first chapters are a good overview of quantum Hall effects. Also it is obviously biased towards Jain's theory of composite fermions (as its title reflects!) and so full of hand-waving arguments to try to justify it.
Not easy to find, I like it though because it covers all the experimental stuff you need to know.
Condensed Matter Field Theory by Altland and Simons. Has a lot of example systems to be explored.
Introduction to Superconductivity by Tinkham. Classic book on superconductivity
General Condensed Matter
In some areas a successor to Ashcroft & Mermin
Condensed matter at low temperatures