If you like to have a book handy for some free time to sit on your couch in the garden and read something, you may want to get both:
On the way through the book Mark not just shows features in FileMaker, but also builds an example with the reader. Great for a bigger to follow this step by step. When you dip deeper later in the book, you get comprehensive guides on topics like ExecuteSQL, which shows the FileMaker specifics there.
While Richard works his way through various features in FileMaker, he has an eye for what can go wrong. For a few ancient features in FileMaker, he simply recommends to not use them for new developers creating newer solutions. As he explains why not to use them, the reader may learn the downsides and concentrate on newer features doing the same job, but better. And of course Richard shows how to do things, especially to keep them future proof and avoid possible trouble later.
Since FileMaker went into agile mode and now releases frequently, the books will get updated regularly with newer content. So we expect to buy fresh copies every year. And even if you know FileMaker well, it may be worth reading them. You always find something new you didn't know!
Have you read them? Your thoughts?