Stage 4 theme music of The Embodiment of Scarlet Devil is named ヴワル魔法図書館 (Voile, the Magic Library). Based on this, a few fans (mostly English-speaking fandom) had considered Voile the name of the Scarlet Devil Mansion's library.
However, this is not canon and ZUN has consistently refered to this library as "Scarlet Devil Mansion's Library" or "The Magic Library" in official works without any mention of "Voile". See also topic #9233.
The library is tended to by Patchouli Knowledge, fairy maids and possibly Koakuma. Kirisame Marisa frequently breaks in to "borrow" (read: steal) books, but occasionally will sit peacefully with Patchouli.
The library is often depicted as a massive tesseract (larger on the inside) or even a pocket dimension attached to the mansion.
This tag implicates library (learn more).