I use the Prowriting Aid website - basically free editing. You can paste sections of your book in and it comes up with all sorts of helpful info about your text - grammar, spelling, repetitions of words (the most useful aspect - I overuse 'just'
, sentence structure etc. Find on Word is good too as it shows all the occurrences of the word in the box on the left.
<Added>Sorry about the icon - didn't mean to wink there.