Why do I feel jealous that my friend is reading one of my favorite books?

From what you write, it seems to me that you aren't jealous but envious.

These two get confused pretty often; basically, when you're envious you'd like to have something another person has, whereas being jealous means you do NOT want another person to have what you have. The concepts are related but not the same.

Anyway, I'm not here to lecture you on word meanings but it seems that you resent your friend having overtaken you. The solution, if such a petty thing (I'm sorry to be so frank but that's what it is) bothers you, is to also read those books and novellas that your friend has read - and then some more, so you'll be ahead of it again.

Another, more healthy alternative (if you ask me), is to try looking at the situation with a more mature view. You have no exclusive rights to read these books (as you very well know yourself) but now that your friend has acquired a liking to that writer, you now share that passion with your friend, which should be a unifying rather than a divisive thing.

Be proud that you were the one opening the door to this fictional world for your friend.

Remember that you will also have her to talk about if you decide to keep reading further novels; that is a good thing!

You question is not really about books but about you inner feelings.

What stops you from reading the books she is now reading?  Why don't you read and discuss them together. Or do you resent that she knows more about them than you do and you have to catch up?

You are in the position your friend was in last winter, when she had to catch up to your knowledge of the book.  Stop resenting her and start reading.

Why? Because you're emotionally immature. Go ask in the psychology forum for verification.

Is it because she is moving ahead of you in some way? Is it that kind of jealousy? It is indeed stupid to feel jealous, but jealousy is still a valid human emotion, so maybe you need to get a bottle of wine and read a different part of the expanded series, decide that only the main trilogy is worthwhile, and tell her so. If you can't move on from jealousy by being vindictive, what good is any of this?

Because you like to say you're a reader who loves books, but you don't really live up to it, whereas your friend does. You don't like that. Oh well. 

Strangers can't answer that for you.  This is the problem with the internet, needing to check everything with "the net" instead of spending some time concentrating on uncomfortable feelings.  Sometimes they just need to rumble around in the back of your mind for a while before you can begin to take them apart and make sense of them.  At any rate having a feeling doesn't oblige you to act on it and that can be good to remember.  I think people can get themselves worked up thinking, "oh no, I have a bad/sad/troubling feeling and now I have to DO SOMETHING about it and that might make people upset with me!" when actually, you don't need to do anything.  It's not an urgent question.  One day you'll be doing something really mundane like watering the lawn and you'll see everything clearly as if spread out on a map, and you'll think, "Huh, so that's why it was bugging me, not the angle I thought it would be but that makes so much sense," and you'll shrug and carry on with your day.  Just let your thoughts take their own time.