Something is only ever a waste if you say it's a waste.
You spent time making this book, correct? Whether it be 5 seconds or 5 weeks, you still devoted some of your time to putting words on a piece of (what will become if it's not already) paper. In this time, you had to think about what you were writing and transferring to your page, and this is revision. The time you spent in making this book is not a waste, because - in some way - it aided your revision process.
I completely agree with this, though you might not use the end product, the process I find very helpful. Not entirely relevant, but in making summaries for biology, or rewriting my notes for maths, though I seldom actually look at the summaries, or care much about the neatness of my new notes, the consolidation I get from analysing what is important in my notes, or redoing the questions, it really helps to get a much stronger grasp on the content. Write a reference to solidify the concepts you've been learning all year, choose to use it or not in the exam, but the time you spend making a reference is never wasted!