It has a deep, musty air, the library at midnight. Students fill its space in daytime but keep themselves to select sections only. It's a curiosity he has noticed since he began teaching, and every year only serves to prove its validity. Why they should restrict themselves to certain parts is not very hard to ascertain. Excepting the occasional reader on par with Hermione Granger, most of them see the library only as a tool to be used and put away. Everything not relevant to their course of study is seen as worthless, hardly worth the time. But it's something more than that as well, an underlying wariness and fear of the knowledge contained within the many shelves. They fear it as landsmen fear the sea, a vast, brimming thing, all depths and tides and pools. He loves it like the ocean, deep and wide enough to lose oneself entirely.
He sequesters himself here when there is time, precious little that there is. Too often enough he uses it like the students, burrowing for information needed for the war. He keeps a secret shelf near an unused corner, a repository for books he has bought (or stolen) on journeys in the search for Tom Riddle. Dangerous documents, the lot of them, and with enough potency to kill unwary users. He dislikes keeping them here, but his office is too vulnerable, and here they will at least be safe should evil befall him.
It's mostly for research, and there's no lie in that, but he's never entirely honest about why he comes here. Of course it's for Harry, and of course it's for the war, but deep down it's also just for him. Friends he has aplenty, colleagues who stoke his admiration, but at the end of the day it's just him again, poor, sad, stupid Albus Dumbledore, darling of the wizarding world and the loneliest man alive. The library is where he submerges himself, shutting out the world that needs him so, until there are only the droplets, and the sound of voices clear as the summer day.
He was such a handsome boy, it really wasn't fair. How was he to withstand the assault, the mind as burning as the body was beautiful, a figure out of his fairest daydreams (and there had been so many, for so long a time, they had always made left him bleeding.) Gellert, who could charm the snakes out of trees, claimed him as if by right, the bride he had long been promised and now could finally wed. Was there ever such a bridegroom?
The books he drowns in help him forget the ugliness of his face, the damage time has wrought. In their pages it is always summer, and the days are endless, and the nights always have arms ready to encircle him, bodies tangled upon the same bed. He can forget he is a pathetic old man dreaming in a children's school. He can forget the only person he ever loved killed hundreds and lies rotting in jail for it. But most of all, he can forget the present faces, and live only for the glory of a vernal blaze.