An interesting science fiction book is Neptune's Brood by Charles Stross. A major structural core of the book is how different colonies trade and the finance models across interstellar distances.

How does a colony finance the initial voyage and investment. It raises the concept of slow money ( 1 slow dollar is the economic output of a professional people for 40 years irc) and fast money for day to day use.

When I read Stross's book I was reminded that Cordwainer Smith in the novel "The Planet Buyer" had a somewhat similar scheme, with the value of something being dependent on where it was. Of course, this being Smith, the concepts were a bit murky.

Not based on speed of light travel, but I always enjoyed the old Andre Norton books that revolved around "tramp steamer" type traders. The Solar Queen series was great.

Adding it to the pile of books to read! Thanks