There are a metric ton of grammar programs out there, and many of them are worth it. Grammarly is one of the most popular, but there are many others.
BUT... as with all things involving computers, remember something. They're not intelligent. Your book may have an overarching storyline, but Grammarly won't care. It'll simply ram your writing through a program that compares it to a variety of standards and then tells you what, according to those standards, is good or bad.
Note, I don't say: What it thinks. Computers don't think. Not like humans do, which means that when I read a passage nailing a single phrase and nod along with the character's words...
Grammarly is going to be nailing you with "this sounds monotonous, would you like to change it?" It'll do it with very official-looking red indicators. Writers, especially new writers can be very intimidated by that, and it's made worse because you're not talking to a real human editor, who can guide you through the process and most importantly, does understand what you're trying to do.
So never forget this--automated editing software can be very good. Just about everyone I know uses it. But it is not a person, and it will never see your book as a book. It will just see it as collections of words, and judge it by a list of rules in the program. Remember that, and don't be afraid to ignore the program.
On the other hand, if Grammarly tells you your sentence is too long, and you notice that the sentence in question is two pages long... Well... Maybe the program has a point. :)
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