I think the important lesson here is making rash promises. I did the same thing with my sister a little while ago, and regretted it. I don’t like to go back on my word, so I think in the future I will be more careful with what I promise people. (Is it wrong that I feel slightly better that you did the same thing, Girl From Mars? 
)
I don’t believe my brother will have mentioned the book to him - he’s only 2 and a half, and the book’s for 10-12 year olds - plus at that age won’t have a lot of conception about a gift promised from an aunt he hasn’t seen in a year. Also as my brother values the signature, I can’t imagine him giving it to a kid who is likely to damage the book. I never mentioned the book either, so I don’t think this is the case.
Wow, quite a lot of emotion here - I don’t really begrudge my nephew the book, I am just regretful I offered it too quickly as it would have been a lovely thing to pass to my kids if I ever had any. In thinking about it I also proabably resent the fact that my brother didn’t point this out to me, as several friends did when they found out I had promised the book already. But that’s by the by, it is promised. I just want to make sure it goes to my nephew, but can see from the above that this might be tricky. I’ve talked to my mum about it (she suggested the option to have it back if he didn’t want it), but other than that, I might just sit down with my brother next time we’re in the same country and discuss the best way to keep it nice for him.
Buy an unsigned copy of the book to give to your nephew. Write a short dedication in it.
Then give the signed copy to the brother with the understanding that it would be passed on. Put a copy of your dedication in it — written on a separate piece of paper — and tuck it inside.
You know, or suspect, that the signed copy will end up in your brother’s hands anyway — and frankly, if it ends up in his hands now, it wouldn’t be a waste of all that effort and possible future value. If you just accept it now instead of worrying over it, you’ll probably feel better in the long run.
It may be quite a pang to give it up like that. On the other hand, it puts you in the position of asking your brother to get some Peter Jackson memorabilia from New Zealand suitable for your children, some day.
I was talking to my friends Grandfather a while ago. He remembers when the New York Yankee’s stopped in town at the long gone local railroad station for dinner sometime in the 1920’s.
He got an autographed baseball from several of the players. Imagine a Dimaggio and Marris and the like signed ball. What did the poor farm kid do with a new baseball when he was a kid? He played with it.
Lost forever.
Keep the book in trust (or I lean toward give it to your brother) until the kiddo grows up until the respectful age of 45 or so.
Grandma earmarked some stuff for each of his children and grandchildren but didn’t really tell anybody what was for who; she didn’t give any of it to any of us unless and until she thought we’d appreciate it.
One of my cousins ended up getting 3 books that were supposed to go to his Dad and two other uncles - but the other uncles don’t appreciate them and my cousin does. I got great-grandpa’s going-to-college trunk when I went away to college; my brother got great-grandpa’s medical school “orla” (the photographs of every classmate and teacher, in one large frame) because his wife went to the same medical school. I also got several pairs of earrings my aunt had left behind when she married; when she tried to claim them back, grandma informed her that since she couldn’t be bothered with taking them, they were grandma’s property by abandonment and therefore she could give it to whomever she wanted. This was, oh, only some 20 years after Auntie’s wedding 
20-ing or so the “keep it for when he understands”.
Step I: Get the real autographed book.
Step II: Buy an ordinary copy of the book.
Step III: Buy tracing paper and some soft pencils.
Step IV: Affix sheet of tracing paper over the signature. Trace the signature. Repeat until you’ve got the signature just so.
Step V: Using a light pencil, replicate the signature on the book from Step II. When it’s perfect, trace over it with permanent ink.
Step VI: Gift the book from Step V now. Hold the book from Step I unitl the kid is old enough to appreciate. When that day comes, give said kid the book from Step I.