If you can keep your head.....: Andrew Norriss, The Touchstone (London: Puffin, 2004).
When Douglas Patterson is contacted by an alien in trouble it's because he is an unusually calm, methodical person. The kind of person who can be trusted not to panic and who tends to believe what he sees and hears until evidence is presented to the contrary. In other words, at the age of (I surmise) 11, Douglas is well on his way to becoming a Heinleinian hero. He reminds me a lot in fact of the hero of Tunnel in the Sky.
But Douglas isn't a fool and doesn't believe the warrior herself, but rather Gedrus, an avatar of the Library to which he can link through a Touchstone the warrior has given him. Gedrus confirms the warrior's story--that her planet is suffering a terrible tyranny and that she is on the way to liberate it--and he decides to help her, hiding her body while it regenerates and soliciting the friendship of Ivo, a rather lonely Hungarian boy at school who wants to build a winning robot for Robot Wars and who eats onion sandwiches and yoghurt.
The warrior leaves and Douglas begins to use his Touchstone. it gives him the answers to his tests, lets him help (cheat) on behalf of Hannah, the lonely and rebellious daughter of the headmaster, and aids him and Ivo in building an utterly lethal robot. And it sets up a plan for him to bring his divorced parents back together.
By the time a Guardian arrives to reclaim the Touchstone Douglas has already figured out that maybe he is using it wrong. One of the reasons I liked this book so much is that the Guardian nudges Douglas but neither explains nor rescues. The epiphany arrives when Douglas celebrates the idea that his parents will soon be back together again and happy. Gedrus is nonplussed. What Douglas had asked for was that they be back together "and everything would be like it was before". Not at all the same thing, as Douglas realises.
What Douglas eventually figures out is that what people want is not what they need. After he and the Guardian have got back the Warrior and the missing Touchstones (which together would have triggered a war*) Douglas finds himself appointed a Guardian. Douglas has learned to ask the right questions, the ones that begin by thinking of long term consequences and causal chains. As we leave him, he is serving his apprenticeship as a number of aliens troop by to ask their questions of Gerdus. Calm and methodical, the epitome of the librarian--which he now is--he is unphased by the weird conformation of his clientele.
*Norriss makes it clear that there is a tyrant on the warrior's planet, and that something Will Be Done, but that WMD is not the way to go.