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The book Ashes of Eden is the first of the Shatnerverse series, it details his last adventure before being lost on the Enterprise B

A part I liked in The Return was that Spock was going through Kirk's files and compiling a list of all his enemies and then through facts and logic was whittling it down to the ones that most likely would have had the means, and motivation and sheer hatred of Kirk to wait over 80 years to attack Veridian 3 and steal his body.

I don't remember the full list, but the Squire of Gothos and Harry Mudd were on it and some others.

Ryker pointed to an entry and said "Khan's dead". Spock said "I know" but did NOT remove Khan from the list.

Against all logic, Spock left a minor probability of Khan somehow being alive as he knew the threat of Khan very well.