Then instead of looking for more books, you might want to have her work on her handwriting and slowing down her thoughts into a real paragraph.
This was DS15, and that is what his teacher in 2nd grade had us do. She said his writing ability did not match his reading ability. Since that summer, we have had teachers comment on how well he writes. We had him do a book report and write in a journal. He loved taking the time to write about the book he had just read. So we slowed down the reading and got him writing more. It has paid off. His Freshman English teacher told us, that he writes on a very mature level, and his vocabulary is outstanding for a 15 year old.
While I believe reading is wonderful, writing is also an essential skill. I think a great idea would be to try to extend the "reading experience" by having your DD do some writing and other related activities. She gets the joy of reading, more practice writing and you can add in other related activities or experiences which can promote higher level thinking skills.
For example, if your DD reads The Wizard of Oz, have her annotate the text (with post it notes) when she comes across a passage she finds particularly enjoyable, or that maybe creates more questions and confusion. In the description of the City of Oz, Baum paints a very vivid picture of what the city looks like and I personally would mark it. Have DD draw a picture of the city and then perhaps write a short paragraph about what she thinks life would be like if she lived in Oz. Or if she marks the passage where the tornado is discussed, do a little research and read a book about how tornadoes form then make a soda bottle tornado as a fun way of reinforcing the tornado "lesson".
As a reading tutor in college, I actually did the above activities with a student as we read the Wizard of Oz. We also created (going through all the steps of the writing process from drafting, editing and publishing) our own Oz Daily Times newspaper that detailed the events of the day the Wicked switch was unfortunately smushed. When we finished the book, we had a WoZ party! My student helped me make "Somewhere over the rainbow" cupcakes (combined the math of cooking with reading as we had to read the recipe), made Witches Brew green lime punch, scarecrow cheese straws, Glinda cotton candy, make your own veggie (tin) man out of celery, tomatoes, raisins a DVD ranch dip joint glue, decorated a bit then watched the movie version. Afterward, we completed a little compare and contrast written activity.
I know this sounds like more involved, teachery type stuff, which it is-I'm a recent Education major graduate, I think it would be a way to extend her love of reading to other subjects and cross curricular activities, while increasing her writing and comprehension skills.
As for assessing her actual reading level, contact the closest college that has a teacher education program. At my college, as part of my reading requirement, we had to work in our reading clinic in campus under the supervision of our professor, while taking a reading assessment class in conjunction with tutoring. They may have a reading clinic that could assess her. Or, if you'd like to donor yourself, buy a copy of Jerry John's "Basic Reading Inventory" or Beverly DeVreis' "Literacy Assessment and Intervention for Classroom Teachers". Both provide a variety of ways to assess reading levels, comprehension, fluency and vocabulary skills.
Another I remembered from school and recently hunted down for my kids was "From the Mixed-Up Files of Mrs. Basil E. Frankweiler." 
