Is the Book of the Dead a primary or secondary source?
Well, that depends on whether this is the book you are writing your essay about, or whether this is the book that you are simply using to research a topic. The definition of a primary source is the main book that you are writing your essay about. The definition of a secondary source is a different book or website or magazine or other source that you will simply use to support your ideas about your primary source.
Because of this, The Book of the Dead could be either a primary source or a secondary source. Are you writing your essay about the Book of the Dead? If so, then it is your primary source. Are you using the Book of the Dead to simply grab a quotation or a support for another idea? If so, then it is your secondary source.
In conclusion, please realize that it is important to understand the difference between primary and secondary sources in order to write your research paper. As your teacher probably mentioned, you need a certain number of secondary sources to support your ideas about your primary source.