I still believe looking up the answer in the back of the book is completely fine. It creates a moment of tension. It invites you to justify in your own head that the answer is right before checking. The cognitive dissonance when you see your answer is wrong and really have to challenge yourself, or ask your neighbour, to see why - is all really valuable.

I just don't think "instant feedback" is as important as we think in mathematics education, and might even rob us of moments to practice mathematical behaviours like justifying, communicating and accommodating. Slow feedback does have benefits.

I am a tech enthusiast to put it mildly. I also taught maths in schools from roughly 2010 to 2020 so saw the iPad/app revolution in my classrooms. Anecdotally, I think it made my lessons and my students worse. Books, paper and each other are the best tools (in my very personal opinion).