As someone who writes a fair few book reviews, I found this article by Aaron Armstrong helpful. He writes:
Book reviews are a strange animal. There’s a lot to consider:
How many words to use
How much summary is needed
How much commentary should you offer
…things like these.
But there’s one factor that, for me, is more important than any of the above: how much time should I spend between reading the book and writing the review.
The answer, of course, is it depends.
Some books require a great deal of time to properly process and critically evaluate. This is work that, very often, can’t be done while you’re reading the book. You need time to work through it all and make sure you’re not making a judgment in the heat of the moment (like when the author writes something that’s embarrassingly stupid, for example).
More important, though, is when you’re reading a great book—when you’re in the middle of it, your fired up, super-excited and ready to give a glowing recommendation. Maybe, though, it’s better to give it a few days, even a few weeks, breathing room.
See if the passion you felt for the book is still there.