This is ALOT different than marrying someone who's divorced, etc, where the child/children have known another mother. There tend to be respect/lack of respect issues w/ a step parent, etc when the other parent is still living. This child will NEVER have to deal w/ this, nor will you or the father.
You raise them as your own, but when time comes/situations arise, you talk about the mother, her character (good parts only), things she liked, liked to do, etc. so that the child at LEAST knows something about his birth mother.
If the three of you decide to put a book together for him, I think that would be GREAT and know that the child will be grateful that you made an attempt to let him "get to know" his birth/biological mother. This book could contain pictures, notes from her friends and family members, etc. This is the type of thing that takes time, thought, etc but something the child will treasure for a lifetime. You'll also gain respect for making that attempt.
If she's one of those who wants to do everything right (a woman after my own heart) very likely she'll make alot of mistakes. Keep the communication lines open and be up for discussing, etc as the child gets older. Lucky her.........to get the priviledge of helping to raise a child who's lost a parent. Parenting IS a priviledge and a HUGE responsibility, but one that carries no greater honor.