Showing book cover in public presentation
No, I can't imagine anyone objecting to free advertising. It is just a visible citation, actually. If you think the people at your talk will want to follow up, you could also provide a list of titles and ISBN numbers to make it easy for people to acquire the books. 

Whether your presentation could be considered a "republication" of the graphics or not, I can't say. Not a lawyer here. But publishers in general would be happy to have their wares pushed like that. If you were putting those graphics into some (printed or online) publication, you would possibly need permission, but the titles etc would be enough for such purpose anyway. 

In any case, in the US, at least, showing the cover or even snippets from the books is probably covered under fair use if this is an academic use. In particular, you aren't reducing the value of the work by showing its cover.