Monad

"Monad" had some characteristics of a magazine, e.g. you could subscribe to it and it had letter pages. However, there were also features that suggested that it was a series of books, i.e. ISBNs and the irregular publication dates. The tip that pushes it more in the direction of books may be the fact that it also was published as cloth hardback (100 copies per issue) and even hardback (4 copies per issue). See the copyright pages for details.