How to join a line and shape... to get rid of pointy corners

Hi all! I'm trying to draw simple shapes that look like books lined up. I start with a rectangle (as the spine), then add some lines to make it look like a book. My problem  is that I have a 3pt stroke on all the shapes and lines. So I'm getting some weird, pointy corners. I can't figure out how to join a line to a rectangle shape (now a trapezoid). I'm attaching a picture to show you how it looks when I draw it (top) and how it looks when I view it as outlines.