"Mr. Zhou appeared from between two aisles near the back door, 'You be careful ...'" - It should be a period instead of a comma.
"He pointed up, and said, 'I do not go far, just upstairs ...'" - There shouldn't be a comma after "up."
"He’d mentioned more than once his move to Novica had been partially based on the low crime rates. 'I’ll be back in a couple of days ...'" - I would move the speech down to a new paragraph because when I read this, at first, I thought "He" was speaking, but it wasn't him, it was Sam.
"' ...the book you ordered will be here the day after tomorrow,' he informed me." - You don't need this dialogue tag. It slows the action and is unnecessary because we know already who is speaking.
"I searched the shadows for any sign of movement, and saw nothing." - You don't need the comma.
"'Don't you know' He smiled slowly ..." - You need a question mark at the end of the speech.
" ...my couch,which also ..." - There is a space needed in here.
" In the end I
went and plugged my laptop back in at my desk to charge ..." - I would take out "went and" because it adds extra, unnecessary words to the text. It is tighter to just say, "In the end, I plugged my laptop in."
This is just a description I wasn't sure about: Sam hearing the bookstore door "screeching" as it opens. I've never heard a door screech before, and it jumped out as an odd description.