It's not that this is bad writing. It's not that there's something compelling about it. It's a decent Grisham effort that builds some suspense. You just need to know what to expect. Witness to a Trial provides some background for and introduces some characters from The Whistler. If you are planning to read The Whistler, Witness to a Trial is worth the time to read it. But if you don't read, Witness you're not going to be missing much. Further, Witness to a Trial is not a stand-alone story.
Bottom line: I can take it or leave it. There are worse ways to spend 99 cents.