Reddit reviews Cheng Tzu's Thirteen Treatises on T'ai Chi Ch'uan
We found 2 Reddit comments about Cheng Tzu's Thirteen Treatises on T'ai Chi Ch'uan. Here are the top ones, ranked by their Reddit score.
I think this book should help.
https://www.amazon.com/Master-Chengs-Method-Taichi-Self-Cultivation/dp/1883319927
If it's just for health then I think it's possible to learn from a good book, the one above was pretty much written with that purpose.
Once you get stronger and defeat the health issues, you can find a teacher to understand the martial art side better.
That book combined with videos of Cheng Man Ching on Youtube and his other book which contains even more principles should get you very far.
The second book is unfortunately not written for beginners though and is very martial-arts orientated, but it has many pages of essays on Taiji which are helpful in understanding the Self Cultivation book even better.
Also this book by Chen Weiming is basically a giant Q&A on Taiji, which should keep you going in the right direction.
https://brennantranslation.wordpress.com/2012/10/27/answering-questions-about-taiji-taiji-da-wen/
It is martial indeed. In his book he actually talks about the different stages of Taijiquan training (he has 3 stages for earth, 3 for man and 3 for heaven), and the last ones are actually spiritual. He adds that he hasn't reached that level though.
I don't practice his style though but I do know his teaching, very popular in the US, he was a great master, very kind and very skillful.