All these books have helped me enormously and I recommend them highly:
1) THE PLOT WHISPERER by Martha Alderson
Provides detailed help with plotting.
There is a workbook that goes with it, but you don't need both.
The plotting exercises helped me. I hung a very long piece of white paper in my hall and I drew a sloping line and wrote down all my plot points. This really helped give me a sense of direction and connect visually to my main character's journey.
2) PAGE BY PAGE by Heather Sellers
Helps when you are unsure. Tells you that it is okay to carve out a space in your life to write. The author tells of setting up a cardboard box in her living room, so she could have her own space to work and find her voice.
3) ON WRITING by Stephen King
Everyone loves this book. King is humorous and explains what works for him. He chronicles his journey and struggles early in his career.
4) THE WRITER'S JOURNEY by Christopher Vogel
The mythic structure for storytellers and screenwriters.
This book has been very influential. Commonly used, but often not fully understood.
What does the hero's journey, anti-hero, call to adventure and refusal of the call really mean?
5) THE CHILDREN'S WRITER'S WORD BOOK, 2nd edition, by Alijandra Mogilner and Tayopa Mogilner
A dictionary by age and word; helps gear vocabulary to the right age group. Very helpful. Though I am not sure that one should follow it always because kids like to read books with interesting vocabulary, sometimes beyond their grade level.
What books have been helpful to you?