USP stands for Unique Selling Proposition (or Unique Selling Point). It is about differentiating your product (in this case a book) from the rest.
Consider this question. Why should someone buy YOUR book? What makes it different from all the other books in your chosen genre? Sadly, if you don't know, then how is your potential reader? So far that's more questions than answers, but let's look at a few things we can consider. In an earlier post I discussed the importance of a book cover. The right cover does sell a book, or at least grabs the attention of a potential reader. So how can you differentiate between your book and others, if both have similarly attractive covers? Most book covers on the Internet are flat two-dimensional images. By producing a 3D cover, and perhaps adding a review, you can make the book stand out. Below are two views of the cover of my book - The Pyramid Legacy. The one on the left is the standard flat two-dimensional image I have to use on Amazon, but the one on the right is a 3D version, complete with a book review, which I use on my website, and any other site which will accept 3D images.
As I'm trying to sell a book, I want it to look as much like a book as possible. I use 3D images of both the current book, and its sequel, in my YouTube book trailer, which is another good marketing media for helping to differentiate between your book and others. (Check out www.simplybookish.com for an inexpensive book trailer,)
Sometimes the title of a book doesn't convey the full story, so one or two short sharp sentences are need as attention grabbing headlines. I use "A sci-fi plot twisting the fibres of history together with those of the future . . ." and "For over four millennia the Great Pyramid of Giza has guarded a secret image; until NOW!" on my website, as an introduction to The Pyramid Legacy. Now, I'm not saying for one minute they are perfect one liners, but I think they do convey the link between the past and the future, together with some level of mystery. By creating even the smallest USP you can help your book stand out from the crowd.