Help Needed!

I went to HOSTS today for an hour and read and did activities with "my" two students.  Unfortunately, the little girl I read with during the second half of my hour nearly refuses to read a book longer than 30 or so pages.  Her reading level has been increasing so she is up to "chapter books", books that are about 70 pages.  I am constantly encouraging her but she reads slowly and so quietly that I can barely hear her.  I know she does it because she doesn't want to read a long book, and I don't know how to help her get past this problem.  Even when I was in high school, my friends would pick books based on the length, and that is a terrible thing to do.  It's like choosing a book by its cover.  If anyone has any suggestions on ways to encourage her and help her without intimidating her, I would greatly appreciate it!  I feel like she will be reading short books for the rest of the school year, when I know she can read longer ones.  The words and concepts are no different than the shorter books, she just gets frustrated that they seem long.  Please help!