In the book Tarzan teaches himself to read with books in his parents cabin. He does this with out any knowledge of spoken language. On page 109 Tarzan leaves a note on the cabin door to the people who just arrived. It reads

"THIS IS THE HOUSE OF TARZAN, KILLER OF BEASTS AND MANY BLACK MEN. DO NOT HARM THE THINGS WHICH ARE TARZAN'S. TARZAN WATCHES.

TARZAN OF THE APES."

Now without any knowledge of spoken language, Tarzan learned the whole written English language by memorization. So how did he learn how to write his name? He knew nothing about sounding out words and there would be no books that showed him how to write his name.