Necronomicon I is a quest item available in Kingdom Come: Deliverance. Finding it is the part of a side quest called Restless Spirit that can be unlocked in Ledetchko village. The game doesn't offer you any help in finding this book. However, you can get it by following the information below.
Necronomicon is kept in Sasau Monastery (that location is presented in the picture above). Don't even think that you will get it from the monks. You will have to steal it. Reach the monastery during a night (e.g. at midnight) and remove all pieces of equipment that create sound before you enter inside. With a bit of a luck you shouldn't come across anyone during your infiltration.
Enter the monastery from the south by crossing the draw bridge. The stairs that lead on the upper balconies are located on the left - they are presented in the picture 1. You have to follow the path presented in the picture 2. Below you can find descriptions of its most notable parts.
Go south once you reach the upper balconies. You come across a door that can be opened without using lockpicks (the same goes for all other doors). Keep going through the adjacent rooms until you reach the room with four big chairs (the picture 1). Here, you can find a chest (the picture 2) that you have to open (sadly, it is secured with a hard lock). Take out Key to forbidden books.
Head east and go through a few empty rooms. After a while you should reach a huge library (the picture 1). Approach the huge cupboard presented in the picture 2. Use the previously acquired key to open it (otherwise you would have to deal with a very hard lock).
Necronomicon I is on one of the upper shelves. Leave the monastery by following the same path.