Is this the book you mean - http://www.shelfari.com/books/16154308/ ??
I don't see any American History tags associated with this book, the tags it shows are :
egypt, esoteric, esoteric poetry, eyewitness, ghost, history, india, initiation, kabala, metaphysics, mysticism, philosophy, physics, qabala, re-possessing, religion, removing haunts, sacred mysteries, science, short story, souls returning, spirits, surviving death
No American History tags there... I am wondering if you were seeing your personal list of tags used on the edit tags page?
To add or edit the tags you personally have associated with a book on your book shelf, the Edit dialog Tags tab presents you with a input box in which to put the tags you want for your (personal shelf) record of the book. Below that input box will be a list of either or both "suggested tags" that other people have used for that book, or "your tags" which is a list of all the tags you've ever used on any books on your bookshelf.
You cannot edit or remove tags someone else associates to the book - only your own personal tags for the books on your personal bookshelf here on Shelfari.
The list of suggested tags/your tags are not associated to the book unless you select them in which case they then appear in the input box above. (You can also type 'new' tag words into that input box, you don't have to select from a list.)