My copy of How to Read Your Opponent’s Cards was obtained from the free section of the library at the Palo Alto Bridge Center at the recommendation of Chris Chung. Check out his channel for some edutainment!
Chapter 2 asks the reader to focus on what was bid and led, and just as importantly what was not. For example, leading a low card generally denies an honor sequence in that suit. It also suggests that the defender also lacked an honor sequence in the other suits as well. The book also heavily emphasized counting total HCP in order to place honors. Using all of this together you can make powerful inferences as early as trick 1. For example, suppose you reach a 25 point game as South after West has openend the bidding with 1S, and West leads a low spade. If both the SA and SK are missing but the SQ and SJ are not, then you can assume they are split, since west would have led that honor sequence if he held both. Since West should have ~12 HCP to open the bidding, knowing that East holds at least 3 from the opening lead, you can now place all the remaining honors with West.
Here are some problems from the book to try (opponents lead K from suits headed by AK). Consider the bidding, the opening lead, the missing HCP, and then plan your play. Try to place as many of the missing honors as you can!