What does the author mean by sleeping cottages?
The quote that you are referencing is from chapter 24 of the book. At the end of chapter 23, Winnie woke up just minutes before midnight. Her plan is to sneak out of the house and help the rest of the Tuck family rescue Mae. Because of the late hour, everybody in the town is asleep for the night. Window shades are drawn closed and/or lights have been turned out. What the author is doing with the quote is using personification to describe the house and the lack of activity within the house. The cottages are "sleeping" because their residents are sleeping. The following sentence helps clarify the image.
"The big glass windows here were lidded eyes that didn't care—that barely saw them, barely gave them back reflections."
The author is saying that the windows resemble eyes. Because the lights are out, it appears that the houses' eyes are closed. They are sleeping cottages. I attached a link to a picture of a quaint little cottage with front facing windows. It really does look like a pair of eyes with the front door as a nose.