The timeline of events seems to have been this:

• Makoto Ueda writes the play "Summer Time Machine Blues"
• Tomihiko Morimi writes the novel "The Tatami Galaxy"
• "Summer Time Machine Blues" is adapted into a film
• "The Tatami Galaxy" is adapted into a TV anime series; Makoto Ueda writes the script, as well as the scripts for other Morimi screen adaptations
• Tomihiko Morimi writes this novel, which is a retelling of "Summer Time Machine Blues" using the characters and setting from "The Tatami Galaxy".

The film "Summer Time Machine Blues" is pointless drivel; that's what makes it good. I think I liked it better than this book. Morimi's protagonist has too much bitterness in him to replicate the fun environment of the film. On the other hand, I do have a certain fondness for "The Tatami Galaxy", so it was nice seeing further adventures of these dubious characters.

Thinking about it, without the context of its source material, it would probably stand okay on its own merits.