Sam read a total of 16 books over 8 months. After belonging to the book club for 21 months, how many books will Sam have read in all? Solve using unit rates.

Sam reads 16 books in 8 months, so his unit rate for reading books is 16 books / 8 months = <<16/8=2>>2 books/month.

Sam has belonged to the book club for 21 months.
At a unit rate of 2 books/month, Sam will have read 2 books/month x 21 months = <<2*21=42>>42 books. Answer: \boxed{42}.