"I'm just wondering if this is customary for published work or if my client happens to be very benevolent."
It's neither customary nor unusual. When I was working as an editor, I often got copies of publications from people I worked with, but I never expected anything. Some liked to do it, and others wouldn't. The biggest problem I had was that a large percentage (about 4 of 5) of my clients were from outside North America, so I'd often get books and other works in languages I can't read. One delightful client sent me a copy of her world-famous book, and then later called to ask if I'd gotten it and if I'd enjoyed it. It was a good thing she called, since the text was in Russian and I couldn't even divine the name of the author from the cover, so I had no idea who to thank for it until she called.
Simply acknowledge it as a kind gesture from a grateful client, and you'll be fine.