I like the idea a hi-fi system with floorstanding speakers IN A LIBRARY!
Seriously though, I think the room is big enough for floorstanders. You may want to consider a large area rug on the floor to help with sound and comfort. Definitely have the speakers facing the long dimension of the room. This will give you a lot more options for speaker placement. With such a long room, you may be able to achive good sound with the speakers 3 to 4 feet (or more???) from the back wall. That way you will be able to walk behind and to the sides of them to get at the books if this is necessary. Some floorstanders have a small footprint and others do not. If you think space will be a problem, go with the floorstanders that have a smaller length and width.
Seriously though, I think the room is big enough for floorstanders. You may want to consider a large area rug on the floor to help with sound and comfort. Definitely have the speakers facing the long dimension of the room. This will give you a lot more options for speaker placement. With such a long room, you may be able to achive good sound with the speakers 3 to 4 feet (or more???) from the back wall. That way you will be able to walk behind and to the sides of them to get at the books if this is necessary. Some floorstanders have a small footprint and others do not. If you think space will be a problem, go with the floorstanders that have a smaller length and width.