At various ages in a child's life... kids go through "changing" values and changing interests... which then manifests itself in "changing" friends. Even adults go through periods of "growing out of" certain friends... whether or not there is a real reason or not, and whether or not it is intentional or not.
There is a GREAT book series for this age set. It is called "American Girl" books. It is great reading for the girl and it also meant to be very kid friendly and a book that can be read with "Mom" as well.
There is a few in particular: "The Feelings Book: The Care & Keeping of Your Emotions (American Girl) " , and, "Friends: Making Them & Keeping Them (American Girl)" and, "What Would You Do? (American Girl Library)",
Or the book: "A Smart Girl's Guide to Friendship Troubles by Patti Kelley Criswell and Angela Martini.
I would really recommend these books... and since your daughter seems to enjoy reading. AND, you can "both" talk about it. It is a real kid friendly series and written in a real nice way and is age-appropriate.
Other than that... make sure your daughter KNOWS that SHE IS A GREAT GIRL... and not to take things like that "personally." Naturally it hurts one's feelings... but the currents of friendships will always change like the tides. So, if she has a good sense of "self" and "confidence" it can help her to weather these things better.
And, there is also the route of her just finding other friends... that are more in line with her interests/personality. Although anytime a friend leaves you, it is sad. Even for any grown up.
Let her know you are always there for her if she wants to talk.... but let her talk freely without critique... I think she probably just needs to "vent" and find a new "rudder" to guide her through this. You don't have to "nag" ... just be a soft place for her to fall... for anything. As she becomes a "teen", you want to build a foundation of trust and comfort for her... so she will always come to you with "problems" and just to talk.
Its so hard this age... I remember going through that too. But I didn't tell my Parents. I just found other friends to bond with. At this age, there are so many "cliques" and so many in and out groups of friends.
- Or perhaps, ask her Teacher how she is doing in school... and if she has observed anything... that may shed some light on this.
I hope the books help. You can get it from www.Amazon.com or any bookstore. On Amazon, you can read the reviews of it.
All the best,
Susan