Its always made its money selling hope and dreams of panda petting paradise to people. Books were just the vehicle to do that.
For most of its history, it made the bulk of its money selling magazines (and in the early days, books) to members of the public, not so much its own members. I think one of the Studies in The Scriptures books was the third highest selling book of all time, at one stage long ago (the bible holding the no 1 spot).
Hell most religions have sold nothing since their inception and have done very well.
Other religions tithe. If you count elders as clergy, Watchtower has a much higher ratio of clergy to adherents than other religions. Other religions do have businesses. SDA has food companies, nursing homes, a University, and goodness knows what else. Catholics have huge property portfolios, schools, hospitals, hospices etc. Other religions are far less micromanaging. Watchtower has to come up with a new "church" service twice a week, whereas Catholics, for example, give the identical church service every 3 years. And to top it off, many Christian religions are starting to struggle.