My oldest (Aspergers) knew right away as he was 8 when diagnosed. My middle son (ASD) we told when my oldest was diagnosed. We didn't use any books just told them. They both did very well with it. I still sometimes wonder how much my middle son understands what we told him. My oldest did struggle a little at first because he didn't understand the different degrees of Autism and thought since they both had the diagnosis it meent he would be like his brother but after explaining the differences he did OK with it.

I highly recommend the book "All Cats Have Asperger's Syndrome". My little one was diagnosed with HFA when her siblings were 5 & 8. This is the book that really helped my older two understand their little sister's condition. I explained to them what the developmental pediatrician had said about the diagnosis being HFA rather than Asperger's because of the speech & language delay but that otherwise the symptoms are similar.

My daughter is classic Aspergers (extremely bright and ahead of the curve academically, struggles socially) but, of course, they don't call it that anymore. I'll see if they have that book at my local library. I'd like to read it before sharing with her. Thanks for the tip!