Many #books , tutorials and articles have been written on wave #theory , but the basic textbook for those wishing to study it for many decades remains "The Complete Course in Elliott Wave Law", co-authored by Robert Prekter and Alfred Frost.
In later reprints in English it may be found under the title "The Elliott Wave Principle. The Key to Understanding the Market. This book can be purchased on the website of Alpina Publishers.
Also some web resources offer to download it for free. Most often these are the sites devoted to trading on Forex markets. For example, there is a pdf-version of the book published in 2001. And on this site you can download an older edition in various formats.
Prekter and Frost's book contains not only theoretical information, but also a lot of illustrations, schematic and visual examples which allow to grasp all the nuances of Elliott Wave Theory.