Guarantees in publishing are usually misplaced. John Grisham and their ilk are the exceptions that prove the rule. A PR firm, book marketer, or anyone else who promises a certain number of book sales is just blowing smoke.
Consider two books, A and B:
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If you’re looking for guarantees in publishing, keep looking.
Which one is more likely to blow up? Take a guess. At the same time, though, each of these (by themselves and collectively) only helps. If you’re looking for guarantees, keep looking.
As with many other things, here’s the secret to book sales: there is no secret. Deal with it. You can, however, do things to increase your chances of success.