No way for me to track sales on books other than my own. My daughter's seem to sell pretty well. As far as I can tell by looking at the number of printings, the books do well.
How to tell is, usually there is a list of numbers at the bottom of the copyright page that looks like this on a first printing: 10 9 8 7 6 5 4 3 2 1
Every time the book goes back to press, the number at the end goes up one, so if the sequence goes like this:
16 15 14 13 12 11 10 9 8 7,
then the book is in its seventh printing.Typically such printings are an additional 10-15,000 copies.
Time it gets to twenty printings, you probably have a quarter million copies shipped. Depends on the initial run, aka laydown. (A direct movie tie-in usually ships more the first run, but doesn't go back to press as much.)
Last time I looked, nobody's books other than mine, my daughter's, or our collaborations had as many reprintings, so I'm guessing ours are still the best-selling ones of the bunch.