Free on Archive.org, Mike Ashley’s overview book The Time Machines : The Story of the Science Fiction Pulp Magazines from the Beginning to 1950 (2000, University of Liverpool Press). I assume the University of Liverpool Press have unlatched it via the ‘Knowledge Unlatched’ programme. Amazon USA wants amazingly high prices for it in print, though Amazon UK has more reasonable prices.

There’s also a follow-on book for the 1950-1970 period.