Microwave on high for about a minute and check one of the pages. The leaves should be warm and still flexible. If they seem quite moist, give them another minute or so. Take the book out of the oven and let it sit until the pages are cool. Be careful not to cook the leaves! If you can smell them they are cooking.
Years ago I read an article about glazing dried leaves using floor cleaner/shiner, like Mop and Glow, which leaves a nice shine that brightens the colors. I still use it, though I have heard of using acrylic medium or spray finish. I just moisten a paper towel with the floor cleaner and smooth a thin layer back and front of each leaf. It just takes a small amount. If you get it too wet, the stuff just beads up on the leaf and dries in spots.
Spread the leaves on a sheet of waxed paper to dry. Some may curl a little, which is usually fine for me, but you can put them back in the phone book overnight for flatter leaves.
So pretty! I scatter them on my table. You can glue them in a sketchbook as well. I have even glued them to fabric and covered with gel medium to use in artwork. The colors do fade over time.
While the bright-colored leaves are the most eye catching, I am partial to some of the deeper colored, more complex colors.
I like to take a few and tuck them into favorite books. They last for years out of the light and are always a lovely surprise to find inside one of my books months later.