Keychain style flashlight or headlamp, LED type, not incandescent: The keychain style is tiny and lightweight, the headlamp style is a bit bulkier but easier to use. You can get these for a couple of bucks or less at the local big box hardware store. The LED ones are lighter, more durable and last considerably longer. Very useful if you need to make a repair after dark! I keep a keychain style one as a backup as I normally have a headlamp in my backpack.
Writer Peter Flax, the former editor-in-chief of Bicycling magazine, rode over 1,000 miles and tested 15 tools for our full-length guide to multi-tools, and he concluded the Topeak Mini 9 is the best for casual cyclists. It’s tiny, it’s light, it’s easier to get some leverage with than other tools that have different designs. It’s not meant for serious wrenching on your bike, but it’s good for on-the-fly adjustments.
I bicycle to and from work to save gas. I don't have a fancy bike, it's just a Schwinn mountain bike that's comfortable and can carry me plus gear but I want to be able to do my own work on it. I needed a book that was simple enough for someone new to bicycle repair and maintenance but could take me through all the upkeep my workhorse needs, and this book does just that. It's well-written and easy to follow.
If you’re going to use the chain whip you just made, you’ll also need a way to get that cassette off. A lockring remover is a must-have; unless you use one of the shady removal methods I’ve seen on the internet—which I wouldn’t necessarily recommend unless you don’t care much about the condition of the wheel after you use them. I recommend Park Tool’s iterations here that cost about $6, but you’ll need to buy the specific one for your brand/type of cassette. (By the way, I’m assuming you already have an adjustable wrench laying around somewhere to use with this tool. If you don’t, well, you’ll need one of those too.)