Colonel Gene Gurney was a U.S. Air Force officer and a prolific author on many different subjects. Although much of his work was concerned with aviation history, he wrote books on several other topics as well.
An excellent source book on the ruling monarchs from Ancient Times to the late 1970s, this book filled a niche before the days of the internet. It provided illustrations of the diverse and changing European monarchies in an encyclopedic form.
A really nice short histories of the rulers of the various kingdoms, including a number that no longer exist. Nice to have a pretty comprehensive list of monarchs. Nice that he gives some of the lesser studied kingdoms a fairly thorough study. A good reference guide for students and armchair historians.