A book talk is a short (30 second - 2 minute) speech designed to encourage someone to read a book, giving an idea of plot, characters, or themes of the book, but, unlike a book report, does not summarize the whole plot or give away the ending or major plot points. In other words, no spoilers! Book talks are traditionally thought of as a tool used by librarians and teachers in classrooms and in library programming.
Scholastic book talk for Cornelia Funke's "Inkheart" (1:40).
In-person book talks have often incorporated props or other audio-visual aids into the talk, especially for younger audiences. Many online video book talks these days do this as well, using images in addition to the "talk." Some even incorporate an aspect of an author interview. This creates something like a hybrid between a traditional book talk and a book trailer.
Book talk/trailer/interview with Brian Selznick for his book "Wonderstruck" (2:10).
A book trailer is different than a traditional book talk in that it uses conventions typical of movie trailers to advertise or promote interest in a particular book, rather than a single person "talking up" a book. They can use any combination of video, still pictures, audio, and text. Like book talks, they tend to be short, typically between 30 seconds and 2 minutes. Like book talks, they should provide plot "teasers," but not "spoilers."
Book trailer for Jon Klassen's "This Is Not My Hat" (0:35).