Actual, useful repair and troubleshooting advice for prewar is best found online, such as this forum.
The John Hubbard book is a product of its time (1980's) and provides very basic maintenance advice and some rudimentary advice for troubleshooting. This is generally common knowledge for most hobbyists today. It severely lacks any true repair or troubleshooting information, but does give an overview of the operational theory of the prewar technology. $25 is generous.
The collection of material by Robert Osterhoff can be a useful utility for part numbers and other primary documentation, but is also largely superseded by online resources. I might pay $60 or so.