Raingear: I use a rain cape, which weighs 8 oz. No need for hefty jacket, pants...
Shoes: leave a pair at work and wear a pair on feet.
Tools: I leave a small multi-tool on the bike in a small bag with tube.
Books: I have become illiterate since started riding a bike, so no need for books.
Water: carry water on the bike, so it doesn't impact the bag. I empty it while riding too, so it gets lighter.