It depends on what your goal is and how your hormonal state is.
If your goal is to conceive, and your hormonal state is balanced, studying from a book or even a friend can be enough for you.
If you suffer from any hormonal disorder you should study with a certified teacher, preferably one that is a fertility expert such a naturopath or Chinese doctor, that way, she can answer your questions along the way and give you more personal guidance.
If your goal is to prevent pregnancy, studying from a book or a friend is definitely not enough. Choosing that way means taking a great risk of having an unwanted pregnancy.
Almost all the women I met along the way who told me, “I practiced the method and got pregnant” admitted that they learned it from a friend or by reading a book or even an article.
Unwanted pregnancy is not something to underestimate. Dealing with an abortion and risks of side effects have a great significance.
A professional teacher that will guide you according to your personal needs is highly important. The goal is to get you to a very high level of understanding the materials and the ability to apply the method successfully.
A book or a friend can’t answer those questions, can’t give you feedback and cannot make sure you understand the tools properly.