There were some ideas that were pure concept art (These would make a coffee table book) and some that had very developed mission documentation, selected crews and training. I hope he distinguishes how far each one actually made it.
I think there were some missions planned (prior to 1979) to at least reboost Skylab and possibly even do more with the NASA's first space station.Post-STS 107 all but the last shuttle mission had a 'Launch on Demand' or 'Contingency Mission' associated with it that would be launched in case a shuttle was damaged on ascent like Columbia was on it's final flight. I suspect those missions were planned in a lot more detail than some of the missions planned in the late 1970s.
Something that is also rife for discussion is the early NASA plans for shuttle flights. It is a bit hard to believe, but even by the late 1970s (maybe up until 1981?), NASA was expecting to ramp up to 50 shuttle flights a year by the mid-1980s. I remember seeing a manifest for the first 5+ years or show that indicated that a lot of them were Spacelab flights. That was rather cheating, because in order to get the per-flight cost down, NASA would have to fund a lot of NASA flights. Where were they going to get the money for all those Spacelab flights?http://spaceflighthistory.blogspot.com/2015/07/what-shuttle-should-have-been-nasas.html
You mean like justifying reusability by putting up a multi-thousand constellation of your own satellites that you then have to find users for?
There was DoD funding in the US (any other agencies?),
I have read both of his previous books on Shuttle and enjoyed them a lot, but for this one, his editor was perhaps not at the top of his game which takes away a bit from the reading experience.