Question: What is an effective and relatively quick way to think of good story starters or ideas that the book could be based around, especially if you have writer's block?Answer:
You might try playing with loglines from existing stories. Here's a link to an article that describes how to do this...
For instance, if you already know what genre you want to write, you can start with a simple twist on an existing story, or perhaps take an idea from a different genre and translate it into your genre.
Then, armed with the new logline or simple story idea you've created (following the article), just start making a list of questions about the idea - any questions will do.
Once you have a long list, brainstorm lists of possible answers to each question.
Choose the answers you like. Then re-write the story idea to include your new choices.
If you play with ideas this way on a regular basis, you will have no shortage of writing ideas.
I prefer this approach to "story starters" or "writing prompts" because with this approach you're choosing the ideas to work with at each stage, so they fit you and your interests better than a story starter someone else wrote.