I've used two different seams for the library bag. In both, you hold two blocks together, and double crochet through the edges of both blocks, crocheting them together to create a seam. The difference is in which sides you hold together as you crochet. In the one above, you put one block on top of another, with right sides together, and then crochet them together. This gives a nice flat seam on the right side.
So, the other way is to put the 2 blocks together, wrong sides together, and double crochet the seam. Then you get a raised seam on the right side as in the picture above, and those below.
This makes a nice outline to the seam, to highlight the edges of the bag.