Clearly his reading list will go on, at your expense, indefinitely.

I suggest presenting an invoice for the cost of the purchased book, or if you pay out of pocket for therapy deducting the cost of the book from his fees.

When he asks, say that this was the workbook that you thought you would be using, deducted from the fees he thought he would be receiving.

Honestly, you want a therapist not a librarian, right?