And of course their first meeting from Picard's POV (the meeting depicted in Oblivion) was not their only meeting prior to TNG; one event over two decades in the past would not have been enough to account for their closeness.
The Buried Age depicts some of the key events in the Picard/Guinan relationship. They've already been good friends for many years at the time of TBA, but the novel's events help explain what there is about their relationship that's so profound to them both in later years.
Thanks for the heads up, Christopher. I knew that someone
had to have touched on that.