Whenever you try to sell a book (either to an agent or an editor at a publishing house) you’ll likely include “comparable titles.” These are meant to quickly convey the story or theme of your novel.
Example: Harry Potter meets John Wick.

But wait! You shouldn’t use movies.
While it can convey what your book is about, you don’t want to compare different media types, especially when you’re making a book comparison. So, first rule, only reference books.
Double wait! You don’t want to compare yourself to the best selling fiction book of all time.
Even if your novel is AMAZING, you shouldn’t assume it’ll be a mega-hit like Harry Potter. That kind of success is super rare, and more to do with luck than actual talent (don’t get me wrong, Harry Potter is entertaining, but a lot of things went right for that to happen.)
Plus, agents and editors will assume your expectations and self-analysis are unrealistic if you say “my novel is the next Harry Potter and Twilight.”

So, rule number two, use recognizable titles, but not mega-best-sellers.
Triple wait! Don’t compare titles from a million different genres.
Twilight and Harry Potter might both be fantasy, but one is romance and the other is action adventure. Don’t make the editor or agent guess the genre–they should know straight away.
So, rule number three, use books in your genre.
The last bit of advice is to show that you’ve got your finger on the beating pulse of the industry. Don’t compare your book to titles that are “old.” And what do I mean by that? I mean, don’t cite a novel more than 10 years past it’s original print date. Hell, you probably shouldn’t use books 5 years since their print date.
Trends are always changing, and what was popular before doesn’t mean it’ll be popular now. If you use comparable titles, make sure they’re newer, decent selling, ones.

To sum it all up:
Only reference books.
Use recognizable titles, but not mega best sellers.
Compare to books in your genre.
Don’t use novels more than 5 years old.
So many rules!! But don’t fret. Once your find 1 or 2 that fit the bill, you’re set. You won’t have to do this exercise in insanity ever again.
Until you write your next novel, that is.