Two of the many books I’m currently reading are:
Outlining Your Novel & Structuring Your Novel. Both books are by K.M. Weiland.
These books have given me a number of ideas that I could have used when writing the first volume of The Ravenwyng Chronicles. I’ve never written an outline before, but I am going to go back and write an outline for volumes one and two based on what I already wrote. I wish I had both these books when I started writing the trilogy.