You can do it on one book, quite easily. But you will realise that when essayists focus on one book, they concentrate on comparing two differing aspects/characters/plots/emotions/narrators within the same book. A friend of mine wrote her English EE on Pride and Prejudice and she focussed on comparing all the Bennett sisters in her essay and "the defining quality" that Austen bestowed upon them. There was still a running comparison, but the comparison was limited to that one book.
Tbh, that comparative aspect is imperative to any good English EE because the clarity and flow of your argument is paramount to getting a good grade and ticking off all the important check boxes. It just becomes easier with certain literary devices to compare two books, but with others you can just as easily focus on one. Most successful essays I've read, however, tend to focus on two works, but that could easily be chalked up to the fact that most candidates prefer to compare two works. At the same time, the 50 Best English A1 EEs has an essay on Feminism in The Lord of the Rings and the imagery of the outdoors in Emma, both topics focussed on one book, or at least, one series.