I wouldn't be surprised if the actual progression went like:
Senior engineers end up with a moderately complex API for various bureaucratic reasons and don't have time to document it well -> Some junior engineer trying to be helpful writes up instructions as he figures out how to use it -> Junior engineer, still trying to be helpful, can't find anywhere good to put these instructions he wrote, so he gets a book published of it -> Rakes in extra money from said book -> Senior engineers see this and think, whoa, that's a nice scheme, but why should this newbie get all the $$$, let's do it more on purpose! -> Next thing you know, all the APIs are actually more complex and everyone's got a book
Senior engineers end up with a moderately complex API for various bureaucratic reasons and don't have time to document it well -> Some junior engineer trying to be helpful writes up instructions as he figures out how to use it -> Junior engineer, still trying to be helpful, can't find anywhere good to put these instructions he wrote, so he gets a book published of it -> Rakes in extra money from said book -> Senior engineers see this and think, whoa, that's a nice scheme, but why should this newbie get all the $$$, let's do it more on purpose! -> Next thing you know, all the APIs are actually more complex and everyone's got a book