Keith Kenney at Sancta Liturgia has a short list of liturgical resources with links to the Internet Archive. The works are in pdf or "DjVu". The latter is something I had not heard of before. It seems to be a more compressed sort of pdf. To give an indication, Edmund Bishop's Liturgica Historica is 53Mb in pdf and 18Mb in DjVu. In the list also is Pierre Battifol's history of the Roman Breviary.