What do you mean by double-sided? Most decent panniers come as 2 separate bags that clip on using locking, quick-release clips (not hook and elastic or velcro). Some come as horse-style saddlebags that drape over the rear rack but these are generally of low grade. You can use the standard panniers together or as singles on one side. In practice it doesnt unbalance the bike.
For everyday use, an external pocket is useful. Some people find internal dividers helpful but they get in the way for me.
Ortleibs are top quality; they make roll-top panniers, very waterproof but getting in and out of ther bags is an event. Flap-top bags are a bit easier to use as everyday luggage.
Books have sharp edges so get a tough, durable material with.
Other strategies for carrying books are a rear rack basket or a handlebar basket.