Put together by Chris Lawson, author of The Art and Science of Oracle Performance Tuning, it presents questions grouped into four categories; three based on difficulty (junor-, mid- and senior-level questions) and one focused on performance tuning. This makes it really easy to choose the right mix of questions for the particular interview you're conducting. Within each category the questions cover numerous topics ranging from database structure to security to recovery to SQL to Oracle architecture to... well, you name it, it's covered. The questions are precise enough, and the subject areas broad enough, to allow the interviewer to easily separate the wheat from the chaff.
For interviewers, this book is a great tool, and it will definitely take some of the stress out of preparing to interview a DBA candidate. But it will be a great tool for interviewees as well - running through a list of questions is a great way to put yourself into the proper frame of mind before an interview. For that matter, it will likely make a good quick reference for the working DBA too - if you don't often deal with certain aspects of an Oracle system (performance tuning, for example) this may be a great way to get answers to common questions in that area quickly.
Bottom line: A welcome addition to the world of Oracle books. Well worth the investment on either side of the interview table, and potentially useful on the DBA's desk as well.