More interesting than captivating. Mostly interesting for the protagonist's expectations of the future (the year 2000) and how they clash with the "reality" he finds there. That sort of retro-futurism is always fun, especially when it's told slowly and methodically. It ends with a brief scene tying up a few loose ends but feels like more should have been explored.

Yes, on the face of it this is one of Heinlein's novels with multiple worlds, but in this case they're the same world, our world, re-experienced through a time loop. It's the grandfather to both Primer and Back to the Future.

The title is interesting. The "door into summer" metaphor is tacked onto the novel at the beginning and end. Don't get me wrong, it's a lovely image, but here Heinlein's writing is distinctly different. It's nicer. He stops looking at the world through a magnifying glass for a little while to relax and describe something in much more confortable terms. I found it a bit of a lurch, and would have preferred him to stick to one or the other style throughout.