First thing's first, I'm going to find as much info as I can on Margaret Davidson for those grade school students and teachers that keep contacting me as if I were her...

Margaret Davidson is a fairly prolific writer, mostly of biographies and non-fiction.  She is perhaps most well-known for her biographical duo: Helen Keller and Helen Keller's Teacher.  I myself have only read these two and Five True Horse Stories.  I remember thinking they were all wonderful.

This author apparently grew up and currently lives in New York with her husband and (as all good authors must ;) her cat.  (Sources: Harcourt School Publishers and Mocha Memoirs)

A List of Titles

Five True Dog Stories with Susanne Suba
Five True Horse Stories
Nine True Dolphin Stories with Roger Wilson
True Animal Stories
Wild Animal Families 
Sea Otters: Little Clowns of the Sea
Dolphins! (Step into Reading) with Sharon Bokoske
Balto, the Dog Who Saved Nome
Helen Keller with Wendy Watson
Helen Keller's Teacher
Louis Braille: The Boy Who Invented Books for the Blind with Janet Compere
I Have a Dream: The Story of Martin Luther King 
Frederick Douglass Fights for Freedom.
The Story of Jackie Robinson: Bravest Man in Baseball
The Story of Thomas Alva Edison, Inventor: The Wizard of Menlo Park with James Watling
The Story of Benjamin Franklin: Amazing American
The Story of Alexander Graham Bell: Inventor of the Telephone with Stephen Marchesi
Adventures of George Washington
The Golda Meir Story 
Famous Lives
Abraham Lincoln edited by Margaret Davidson
Jacques Cousteau: A Biography 
Guru Nanak's Birthday (The Living Festivals Series)
Shrove Tuesday, Ash Wednesday and Mardi Gras (The Living Festivals Series) 

Make Believe It's You, A Choose Your Own Adventure Series:
No. 1 - The Adventures of Puss in Boots: Starring You with Carson Davidson
No. 2 - Snow White and the Seven Dwarfs with Carson Davidson
No. 3 - The Adventures of Thumbelina: Starring You with Carson Davidson

To find these books you could try searching Amazon.com or Alibris.com as some of the more obscure titles (such as Guru Nanak's Birthday) appear to be out of print.

Contact

If you would like to try writing to Mrs. Davidson, I suggest sending a letter either through Scholastic or RandomHouse publishers.  RandomHouse gives this method of address to attempt reaching their authors:

Margaret Davidson
c/o Children's Publicity 
1745 Broadway 
New York, NY 10019