Attachment Book?

So I met a man who has adopted with his wife two children of relatives. One is older now and seems to have adapted well (8 year old girl). The other was a more recent adoption and I believe the boy is 4 years old. He is showing classic attachment issues (of course I've never met the boy and have only talked to the father twice).

Of course attachment disorders came up in our conversation and I mentioned some local support groups and attachment therapists. The man seemed intrigued, but not enough to take any numbers. Today he sent me an e-mail and asked me:

"Also, out of all the choices available, what would you suggest as the first book or resource to read on the area of rejecting new parents?
-IF you had only one to choose..."

Now my kids are MUCH older and I do not want to scare him with Nancy Thomas. It didn't sound like the child was that severe. I do think the child needs an attachment therapist, and because this was a relative adoption the parents have had NO training whatsoever.

So what would you recommend for the parent of a young 4 year old with obvious attachment issues - he's been with the family for less than a year?