A book I downloaded is in my library, but it won't sync to my iPad. Sometimes I never have trouble syncing and other times it doesn't work. I used to get a different window to sync to my iPad and today I can't find it.

I book I downloaded today is in my library, but it won't sync to my iPad. Sometimes I can sync very easily and other times not. When I downloaded my last update I got a window that had devices and I clicked iPad and then I chose sync to my iPad or something like that. Now I can't find that window. It's very frustrating every time I find a book and then can't sync it to my iPad. Can anyone figure out what my problem is? Do I need another update to iTunes? Thank you!