Okay, this one is a longshot since I've been looking for this fic for months, including a post on Seeking Spike, and have had no luck so far, but you never know so here it goes:
After Spike gets chipped and has moved into his crypt, Buffy has noticed some changes in him. He's quieter, more introspective, almost reverting more into William where he's become nearly a perfect balance with his demon and it makes Buffy goes to Giles with questions. When she's unsatisfied with his answers, she starts doing her own research and eventually comes across a book (I think, it could have been different sections in several books) that explains how the older vampires get, the greater a chance they have in regaining their humanity and possibly even becoming allies for the side of Light, how some are able to evolve beyond their demons and maybe even actively seek redemption, without the benefit of a soul. Horrified by this, it throws Buffy's whole world view into question, she confronts Giles about what she's found, and eventually starts trying to help Spike with what he's going through.
Seriously, if anybody has ANY idea about this one, please let me know because I would really love to read it again. | 
