I am currently re-doing a book of tactical exercises. I have not seen some of the exercises for longer than a year, and when I look at the position at first it seems new and I see nothing, until it clicks and I remember the solution. It is not through calculation, I just stumble upon it and remember it. I of course verify my "guess" by calculation, but once one know it works it is not hard.
I was wondering whether it is even worth going through a book if this is how I solve about 95% of the exercises, it feels a little like cheating and I should do new exercises. But then I think: Is this not how it is supposed to work? If a motif appears in my game, I will get the same feeling that something is there, and that is what the point of my training was, and by going through the book again I will reinforce the patterns.