When Lenormand started to take off on AT years ago, the learning courses were either the book by Sylvie Steinbach "The Secrets of the Lenormand Oracle" or Treppner's Course. It was really either / or and people on AT recommended you pick one line of thinking and stick with as both their viewpoints and angles were different.
I bought Treppner's Course because a friend on AT recommended it to me personally and this was about 4 years ago. I really love it but it is lengthy and sometimes the German translation to English is a bit dodgy. My most used part of her course is the PDF of the combinations and the one which gives an overview of the cards. I have used those so much in my learning! They have been my #1 resource in my lenormand learning.
Now I have formed my own opinions and don't always use Treppner's view but it gave me a starting point. I think also there are so many more resources now for learning. As well as those two, there is Rana George's book and Andy Boroveshengra's book "Thirty Six Cards". Many others on tops of that.
Treppner gave me a great foundation for learning Lenormand and reference PDF's to help. I think Andy's book is the best for learning the GT and more advanced things after that. I would still recommend Treppner's course though to people who want to learn Lenormand