I've looked everywhere and can't find any hay or barns

It's on the northern road of Candlekeep just west of the Temple of Oghma (IIRC).

I'm looking at a map right now and it loos like it's near the northern wall in an open storage area.

Once you see the hay, there should be a little clickable region. Click on it and its inventory will have the book.

Once you get to the hay press the tab button and the clickable area will become highlighted in blue.

This only works if you have Tutu or BGT installed. BG1 and even vanilla SoA don't allow you to highlight items or containers, that was added with ToB.

Pressing the L key should give the x, y coordinates of the mouse pointer.

Look around the area (x=3250, y=290) which is close to Dreppin.

Well, holy crap, it turns out the brick wall is actually some square hay, I've never seen hay packed into cubes before.

Thanks everyone, I found it! I'm loving this game now. I thought this quest was bugged.