This is a practical course in memory techniques. O'Brian was, at the time of writing, World Memory Champion, and had been for many years. He shares his techniques for great memory and runs you through several practical exercises. He talks about how he developed his own memory, little tricks for the things that always give you trouble, like the spelling of certain words, and the cool thing is that he techniques really work.
I listened to this on my morning commute, so once we hit the more advanced exercises, I was a bit lost (had to keep safe in trafic, after all), but even the early exercises I was able to do helped significantly.
The book is well written, and he's an excellent reader - always iffy when the auhor reads. I recommend this book to anyone who is interested in improving their memory.