"How to Be Twice As Smart" by Scott Witt. Memory, math and other techniques.
"How to Develop a Perfect Memory" by Dominic O'Brien. Synthetic and recent. Mainly focused on practical issues. Uses the DominicSystem.
"Mind and Memory Training" by Ernest Wood. Deep discussion of how to improve memory, concentration, reading.
"You Can Remember" by Dr. Bruno Furst. First published in the 1940s. Mostly practical, but includes some card memorization tricks.
"Your Memory - how it works and how to improve it" by Kenneth Higbee. Written by a psychology professor, this book is more academic than most other other books on the market, quoting plenty of research findings.
I wanted to add comparisons of different MemorySystems and figured this would be a good link point. --- JohnLaviolette