This book will teach you all you need to know. The big problem is that, unless you continue practising all the time, you will forget almost all you have learnt. You can, of course, re-read & re-learn the subject using the book.
The same problem applies to the classroom course, which means you have wasted the course fees, and associated costs (e.g. course pack). Contrary to popular views, you never handle a sextant, unless the Tutor, or another student, brings one to the class. If so, you can practice handling it by using the building corridor, or classroom floor, wall, & ceiling. Practice outside is not part of the course.