I have three that I go back to regularly. Two are by Mary Griffith...The Unschooling Handbook and The Homeschooling Handbook. The other is Home Learning Year by Year by Rebecca Rupp. The Rupp book is wonderful for having an idea about what your children "should"(used loosely) know each year. Not only does she say what the concepts are but she also gives great suggestions for resources to teach each concept.
One other new one that I am on my way to loving is called Great Books for Girls by Kathleen Odean (the chair for the 2002 Newbery Award Committee). The book gives a brief synopsis of each book and they are listed in alphabetical order. I have been awesomely impressed with the selections...great strong female characters. Just a quick "by the way", one of Emily's favorites was You Forgot Your Skirt, Amelia Bloomer by Shana Corey. It is a great, funny way to expose your girl to women's rights, or our struggle for rights, back in the mid-1850's. Highly, highly recommend it!
I also think she has a Great Books for Boys for those of you with boys only.