Title: Tou no Naka no Himegimi (Princess in the Tower, though the official English title is Lady in the Tower)
Circle: Studio Canopus (Yamada Mario)
Fandom: Sailor Moon

This is another great Sailor Moon doujinshi from Studio Canopus (who also did Shi ga Futari wo Wakatsu Made and Hoshi ni Negai wo). The creator, Yamada Mario, also did a short for the Colorful Moon 8 anthology, "Lonely Tropical Fish," which has been scanlated by Lililicious. Lililicious has also scanlated another short Studio Canpus doujinshi, The Little Mermaid.

The scans from the first Tou book come from kaifuu.com, which has several Sailor Moon doujinshi available for download in the "Comics and Manga" section (the site uses .cbr format for the files; if you don't have a program like CDisplay that you use for .cbr files, just change the file extensions to .rar and open them with a rar program like WinRar). The scans of the second and third parts were sent to me by a very kind fellow doujinshi fan, Ever, and seem to have originated from the yamibo forum.
If you haven't seen Haruka's flashback episode (episode 106; it's from the third season of the anime), you might want to skim through a summary of it before reading this doujinshi.

Part 1
Part 2 (missing pages added 8/10)
Part 3