Have you seen the Lexile Framework for Reading website? It's a free site that allows you to search for books at a particular grade level as well as searching for subject matter/style (science, romance, history, children's book award winners, etc.):
http://www.lexile.com/findabook/StudentInfo.aspxI don't know if the list is limited to only one publisher's books. It's possible. But the site at least gives you a good place to start looking. It's where I start most of my book searches, even before I hit my local library.
I don't know if you can find books with comprehension questions using this site, but I rather doubt it. I don't think there's any way to specify that. Some editions of a book will include questions and other editions of the same book will not. It's pretty hit or miss (mostly miss!).
One way around this is to read the books together and discuss them, asking your own comprehension questions. This is what we do, since books that come with questions are so few and far between.
Maybe that helps?