Do the books come with patterned paper already in them, or no? If not, how exactly do you go about putting patterned paper on/over the pages?
No, books don't generally come with patterned paper unless you're buying a kit that comes with a lot of items.
How you add patterned paper depends on your book - there are several different styles and sizes.
I currently have a scrapbook that is all white plain paper, white hard front and back covers, and a purple spiral spine. I think this would look nice.. but it's so plain. It's just too white. I wanted patterned paper on it so it would look really inviting and unique. Would I just cut the paper to size and glue it onto the page in the book, or would that not be acceptable?
I'm more of a visual person...can you post a photo of your book in your gallery so we can see it? Like I said, there are several different kinds of books. Depending on what kind you have, instructions on how to alter it will be different.