I don't know who the publisher was, but it doesn't sound like O'Reilly. When I worked for them (in the 2000s) our mantra was that the right size for a book is h

I don't know who the publisher was, but it doesn't sound like O'Reilly. When I worked for them (in the 2000s) our mantra was that the right size for a book is however long it has to be to say what has to be said. There's a minimum length of around 120-150 pages, below which it's hard to have a visible cover on the spine, but there's no inherent benefit to 600 page books versus 250. With ebooks, of course, there's no such thing as a minimum size: it's whatever there's a market for.