Puzzle training: rotating tactical themes or exhausting each one? I agree that many people like it more when chess puzzles do not give a clue on the tactical theme. That is not my question, and I would like to focus on how to study a collection of chess puzzles classified by themes. I have one of those books, with one chapter devoted to each theme. In each chapter the difficulty is supposed to increase.

When I was preparing for undergraduate math exams, I used to rotate the exercises: one exercise per chapter, instead of devoting myself to one chapter exhaustively until jumping to the next one. That way, I was reviewing all the material in each session and was exposed to a bit of everything all the time. This approach has always served me well.

So I am doing the 1st chess puzzle in each chapter and, after reaching the end of the book, I go back to the beginning and solve the 2nd problem of each chapter and so on.

However, authors do not seem to have designed chess puzzle books for this approach. Sometimes they may even give detailed instructions like "look at each puzzle for 5 minutes and then look at the solution", "try to be exposed to 20 puzzles per day in order to develop a sense of pattern recognition" but there is no mention to this rotation approach I think I should use.

Is it better to focus on one tactical motive for a while instead?
Whatever works best for you.

I guess for a beginner it does make sense to see / study different variations of the same theme first. If you feel that it is too easy studying one chapter at a time, i.e. if you can spot the solution within a few seconds, it would make sense to mix it up a bit. You can do this by randomly picking problems from your book or online at sites like chesstempo or lichess or many others that have chess puzzles and that automatically adapt to your strength.

Once you get to more advanced puzzles you will see that the solution usually consists of several motives, e.g. a "clearance" followed by a "fork". At that stage you will not run into the problem of being stuck with one motif.