You don't need to do any preparation for this course.
My commitment is to cut through the clutter from all the various books you could read...and ensure
that at the end of 15 hours of instruction
you can navigate across an ocean with a sextant in your hand and
the salt spray in your teeth.
The danger is that you will read a book, get confused, despair of ever
learning, and arrive at the course demoralized.
Further, you don't have to get very deep into celestial navigation before you realize that every navigator with a copy of MS Word has his own data worksheet format...and I am no exception. If you start getting into some other author's head, you are going to find it a little difficult to orient yourself to how I think about celestial...and what my worksheet looks like.
That said, I do have two books that I am willing to recommend to you. One is a book that is out of print now...but there are still plenty of used copies available from Amazon. I used it to learn celestial navigation 30 years ago...and I still think it is the best: John P. Budlong, Sky and Sextant: Practical Celestial Navigation . You can get a 2nd hand copy for less than $11 from Amazon.ca here.
There are some (allegedly) new copies floating about, but this book is definitely out of print. So be careful. It might be safer to buy a used copy for $3.78 (plus $6.49 for shipping).
Budlong's explanations are clear and concise...and his worksheets were the starting point for the ones I have evolved. So there will be less of a mental shock to you to make the move from Budlong-to-Bob than to go from AnybodyElse-to-Bob.
This book the best of the currently published books once you know what is going on...but is not what I want you to use for advance preparation. It will help the student who says, "I understand this today, but this whole class was only 15 hours long. I will need more practice to really set it all in my mind." It has a number of excellent practice problems you may work out. I will bring several copies along with me during the course proper and give you an opportunity to purchase one at cost.
Bottom line: I am committed to teaching you to navigate by the stars in 15 hours. You don't need to worry about advance preparation.
That said, if you really WANT to do some advance work for the course, pick up a copy of Sky and Sextant: Practical Celestial Navigation, by John P. Budlong.