How do you call a book trailer that's going to release after the book was published?

Well, I personally would probably call it by saying "HERE, Trailer, Trailer, Trailer. C'mere, boy! Who's a good boy?" But that's just how I do. 

Trailer are the advertisements for new and soon to be released movies that are shown before the movie you are about to watch behind. (What a long winded sentence!)

If the advertisement for a new or about to be released book is aired, it is a publication or pre-publication announcement. It's an ad, just like the movie trailer is a pre-release announcement.

If you are asking about the paragraphs on the book jacket or the back cover that should entice you to buy the book, those are called "blurbs". I don't know why, but it can probably be "Googled".