It is a good idea to keep a list of topics you can fall back on when you want (or need) to post and not sure what to write about. I'll share my list with you, and for authors I have a few suggestions for you too.
1. Themed list
This is always my go to topic when I don't know what to blog about. It works for both my tips posts as well as book posts. Numbers in post titles do well so that is a bonus. 5 books to read before watching the movie, 10 hottest book boyfriends. But you don't have to have a number and can do any theme - specific genre, writing tips, top blogging organization apps, favorite blogs to read, etc.
2. A game
You might think a game would be difficult to come up with, but it can be pretty simple. Post the first few lines of books and ask for people to guess the story. Guess the character - give descriptions of characters. Crop portions of book covers to have people guess the title. It doesn't have to have a prize for people to play.
Check out Goodreads or Pinterest boards to find famous quotes. You can either just post the quote as is, create a graphic with the quote (you can make them pretty easy with something like PowerPoint), write why you like the quote or what it says to you.
4. Feature a book or author
Pull a book that has been waiting a while in your TBR pile - what made you choose it?. Or highlight a favorite author - why do you like the author? What books have she/he written?
- Share a snippet from your work in progress
- Give background on a character from one of your books
- Write a deleted scene or a scene from another character's point of view