Erik had found a child's story book for Jane to convince her she was wrong. That it was all a delusion. He was almost embarrassed to recall, now.
Later, after the battle with the Destroyer, he had returned to his home and dug through an academic's lifetime of books. S.H.I.E.L.D. had been in a hurry, but Erik had simply insisted it was something he needed. A decent bookshop would probably have furnished a new copy - and if he pushed there were passages he knew by heart - but it would not have been the same. The thin, battered and dog-eared book had belonged to his father; had been read to him as a child. So it was that Dr. Erik Selvig, astrophysicist, had in place of honour in his laboratory that classic of his ancestors 'The Prose Edda'.
It had been saved from destruction by merest chance. Erik had been flicking through the passage regarding the Rainbow Bridge - the Bifrost - when the Tesseract had awoken. Absently, he had tucked the precious thing in a pocket where it had remained - unrecalled, he shuddered to think what Loki would have done had he found it - throughout his captivity.
So it was that, tired at last of the God of Thunder's swaggering, Erik had handed Thor that history of his home and people, commenting, "It isn't that clear cut at all, and you know it." Thor had been immediately absorbed and strangely silent - for a while. Then he had begun to mutter, meaningless half-phrases: "Mother would never..." and "This can't be how you remember it!" and "He can't think we meant...".
When Thor reached for a pen, Selvig's patience snapped. He snatched the precious book. "If you want to re-edit it, I'll buy you your own copy!"
Thor wanted to apologise. He hadn't understood the concept of trading cards, but he had seen how important they had seemed to Coulson, and how grateful he had been for the mark of the human, Steve Rogers.
The next time the professor had opened his Prose Edda, he had found the fly leaf covered in runes. Below these the translation, in the same incongruous blue biro: "Erik Selvig, sworn friend of Thor Odinson of Asgard, with gratitude."
There really was nothing you could say to that.