Do I need a sewing machine for this?
You don't need one, but it's much faster and usually more accurate to use one. I realize that everything used to be hand done, but there's not a chance I would tackle a quilt without a machine.Would it be easier to just craft a duvet cover from all these materials or make a quilt than to do it this way?
For me it would be easier to make something new than to add to an existing duvet. If you went the quilt route you likely wouldn't need to applique anything unless your embroidered designs are really close together.Any tips for a wayward embroiderer taking a left turn in to quilting?
Welcome to the dark side.
I think the most important thing is to take the time to cut accurately. When things are the right size they fit together nicely and your finished project will look better. You can always fudge an incorrect piece when sewing, but cutting is really where it all starts. If you are able to do it you should invest in a rotary cutter, mat, and cutting grid. If that's not possible right now you can always use templates for marking and cutting out your pieces. Most quilt directions will say to use 1/4" seams, but it's more important to be consistent than always at 1/4" IMHO. I always use the same mark on my machine, and even though it's not 1/4" it's OK because I use the same line throughout the whole project.
What's a good beginners quilting book?
This is one area I can't help you at all. I was taught to quilt when I was about 9 or 10 by family members, so I've never used books for learning technique. Go to your library to see what they've got. If you really like one you can go out and buy it, and if it ends up being not so great you haven't spent any money. Best of luck, and be sure to post what you end up doing.