Reddit mentions of The Door into Summer

https://www.amazon.com/Door-into-Summer-Robert-Heinlein/dp/0345330129/ref=sr_1_1?ie=UTF8&qid=1519432691&sr=8-1&keywords=heinlein+door+summer

I would have picked another Heinlein title, although Stranger was the best-seller. In my opinion, Door Into Summer was far better.

Kitty's just looking for the door into summer

If you have never read The Door Into Summer by Heinlein, you should check it out.

Just a suggestion for reading: Shine, an anthology of optimistic sf.

EDIT: some other optimistic sf

• The Door Into Summer by Robert Heinlein
  
• Way Station by Clifford D. Simak
  
  Any other suggestions anyone?

Maybe it is one of these
http://www.amazon.com/Door-into-Summer-Robert-Heinlein/dp/0345330129