This is book 25 of the "Spenser" series.

This note contains spoilers for the book.

I didn't recognize this at first; the blurb focuses on the guy being Susan's ex, and that didn't access it for me. But when I realized it was about money laundering through charitable fund-raisers, I recognized it. There are, however, some very nice bits that do have to do with the guy being Susan's ex.

Most amusingly, it's all a mistake; he never really wanted Spenser's help anyway.