Using photos in stock photo for commercial use.

First: change your username here as not to include your e-mail. That's dangerous, as spammers and scammers will contact you.

Second, yes, you can do that. That is what stock is for. And that has nothing to do with “Fair Use”, as that is a term, describing a practice of using copyrighted material, without needing the prior authorization of the copyright owner for anything that falls under the “Fair Use” dispositions. Using copyrighted material for a book cover is not fair use and requires a corresponding licence. You get that licence when you buy a stock asset.

Look here for more information on licensing: https://community.adobe.com/t5/stock/links-for-licensing-terms/td-p/11366788

(my “fair use” in the disclaimer is just that I think that the use is correct because you paid, and the copyright owner got paid!)
(Disclaimer: As always with licensing, this is my interpretation of the rules. I think they are correct and advice is based on reading and interpreting the licence terms and on fair use for both the buyer and the artist/stock company, but I cannot rule out that my interpretation is wrong. I'm not an Adobe employee).