The concept is simple: Peter Seibel sat down with 15 different programmers (including such greats as Brendan Eich, and Don Knuth) and asked them the kind of questions that a fellow developer would ask. The interviews were insightful, and in-depth: giving a glimpse into the history of each person, and exploring their personal views on everything from the dangers of C to the usefulness of debugging tools more advance than simple print statements.
Software is often a solitary and opaque profession, and this is the next best thing to personally sitting down with these computer legends to pick their brains. If you are involved in software development (programmer, architect, analyst, whatever), I highly recommend this book.