You apply the book to an item that you'd like to have that particular feature. For example, if you got a book with sharpness, you can put it in an anvil with a sword; the sword would then have sharpness on it.
In the create new world menu, just scroll down until you find the "world type" area. Then click on where it says "infinite" and select "flat" from the drop down...
Unfortunately, no. As of the time this answer is written (Dec 1, 2020), the Bedrock Edition (formerly known as Pocket Edition) of Minecraft still doesn't allow for custom Superflat worlds....
