Stack Whackers is a story written by shukyou. It ran in Issue 44 and can be found at http://s2b2.livejournal.com/257468.html and http://www.shousetsubangbang.com/mirror/stack-whackers/.
A voyeur and an exhibitionist mutually discover this about themselves.
"The what?" I asked.
From fail-fandomanon.dreamwidth.org: "College librarian encounters hot frat boy in stacks with his dick out. First person voyeurism kink story is cute."
From pinboard.in: "the one with the exhibitionism/voyeurism in the library"
Stack Whackers has been featured at GoodReads.
This story is available for download/purchase on Smashwords.