Well, writers should read lots of things, particularly outside of their genre, however, this list is about fantasy books. There are 5 in this list, but I'm quite sure there are more to come. Feel free to suggest others. I've linked them so you can read a brief outline if you are so inclined.
1. "Playing Beatie Bow" by Ruth Park
This book should be read for a number of reasons (though one of the standout reasons is that the film is "not great"). The reason this book heads my list is because it has fantastic characterisation, and the blend between reality and fantasy is pretty seamless. It is also one of the few time travel stories I have read that does not manage to tie itself into an incomprehensible knot. Very good read.
2. "The Redemption of Althalus" by David Eddings
Personally, I love pretty much everything by David Eddings, up until "The Redemption of Althalus", but I think in this book he has really mastered his style. Characters, plots and settings weave together beautifully in this book (which fortunately is a stand alone novel). The only down side of the story is that if you have read many of David Eddings' earlier works, there are many familiar overtones to this story.
3. "100 years of Solitude" by Gabriel Garcia Marquez
This is not necessarily a fantasy book. This one is described using the phrase 'magical realism' where imagination and reality are moulded together in such a way that it creates something truly moving and very amazing. With its whimsical use of time and fascinating characters, this book is truly a page turner.
4. "Sorcerer's Ward" by Barbara Hambly
I know straight away that fans of Barbara Hambly are going to insist that this is not the best example of her work, however I found that this book creates a magical world in so much detail that I really believed it, while I was reading. In the attention to detail of every outfit and conversation and building, there are no moments where you wonder if an event would really take place in the world. This book is here because it is a case of a truly masterful use of setting.
5. "A Modern Magician" by Robert Weinberg
Because all the other books in this list take themselves reasonably seriously, I included this example of more light hearted fare. A very entertaining tale, set in the modern world. Very action focused, the characters are not as well developed as in some of the other examples provided, but it is great fun to read.
I would love to know what you think of some of these books, and what you think should be on the next list. Have fun reading.