Here is a (legally accessible) sample chapter from the book:
http://press.princeton.edu/chapters/s8963.pdf
What this extract from the book illustrates is how very well Gray and Corbally write, and it is, in itself, a very valuable 27 Page Essay on the history of spectral classification.
If you need more information about the contents, the style, and the readership level, you can also access some of this book at Google Books:
http://books.google.com.au/books?id=...cation&f=false