I think it is speculation. I looked for a reference in the text, and could not find a hint of what Jaqen/Pate is doing. He may even want one of those glass candles for the FM. He may be after all "knowledge" he can get.
I think some speculate based on the books Sam brings for Maester Aemon, which may or may not have a dragon book among the scrolls. But Sam has to sell them to the Summer Islanders for passage, but someone reassured Sam they would be sold back to the citadel.
The only dragon book I recall is the one Tyrion read on the way to the Wall from Winterfell, yes? Tyrion borrowed the book with Ned's permission. Maybe the book is among those Aemon selected to gift to the citadel if Tyrion finished it and left it at the wall.
I could very well be wrong. I am no good with timelines.