Informal"" here means that Willy Ley has introduced much off-beat and fascinating information from the history of astronomy into a broad survey of historical developments in this basic science. Following a chronological path at the start, he later adopts a stimulating topical approach, both of which reveal what a startling amount of research, and no doubt enjoyment, went into putting this book together. All basic facts are here plus quite a bit of biographical information about many astronomers and other technical information in appendix.