footnote question Wanda Sellar's book

1
Hi
I am reading Wanda Sellar's Intro to Mecical Astrology and in her footnotes she refers to " ibid " . Anyone able to enlighten me to as to what or whom she is refering.In her list of references in the back of the book it is not listed.It is a large work as one footnote has "ibid p 918."
So far I really like this book.I am in the holistic health field and have been disappointed in most new books written on the subject but this will be the one I will definitly refer to.
Thanks for the review Deb
Blessings
Azcelt