I don't think we have discussed this in a systematic manner yet. Many tactics books, certainly by the big British publishers, try to cover a very wide range of difficulty, probably to appeal to a wide market. But that makes them not perfectly suited to any particular reader if you think about it!

I have a personal reason for starting this topic too; I'm looking for the perfect tactics book to use while travelling or otherwise away from my beloved CT-Art program. The criteria I'm looking for, in order of priority:

- Portable, not too big and thick.*
- Focus on middlegame tactics, though I can live with a few endgame positions
- Difficulty between 2000 and 2300; solvable without a board
- A narrow range of difficulty, i.e. not many pages "wasted" on either simple 2-move checkmates or award-winning Shirov combinations.
- An effort to select and analyse "new" problems, not the same ones used by everybody else
- Not too many errors in the solutions

Examples of good "close encounters" already in my library, and what's "wrong" with them: 

Test Your Chess IQ: GM Challenge by Livshitz 
- some errors in the solutions. 
- one of few tactics books I have with a rating system, which makes me reluctant to use it "merely" for random travel solving.

Combinational Motifs by Blokh
- range of difficulty a bit too wide
- it just duplicates the CT-Art problem set that I'm already using

Forcing Chess Moves by Hertan 
- just too big and heavy 
- more space given to "study material" than to exercises

Any suggestions appreciated.

* The small endgame study booklets by Russian Chess House are great; why doesn't anybody do the same with middlegame tactics?!