I was considering buying a version of the Sefer Yetsirah, but wondering how I should treat it. It's not the Tanach, so I suppose it probably doesn't require being as careful. Even though my question is specifically about the Sefer Yetsirah, I would also like to learn how one should treat commentaries, editions of the Talmud, Zohar, Shulchan Aruch, even textbooks on Jewish history, etc.
Generally, how you treat a book is not based on its relative Kedusha level, but rather if Gd's name is in the book or not.
The relative Kadosh levels of books only really comes into affect when you are stacking books ontop of eachother, or placing them in a bookshelf. (See Shulhan ARuch Yoreh Deah 282:19 for more details)
But how you treat the book itself, depends entirely on if Gd's name is in the book, or if it's from tanach.
All Torah works are considered holy and must be treated with respect, regardless of the specific genre (halacha, Talmud, kabbalah, etc.), even if the name of God is not mentioned in them. (This is true even if they are not written in Hebrew.)
The only difference between Torah works is that Biblical books are should not be placed underneath non-Biblical works, and Chumashim should not be placed underneath any other books, including other Bilical books.