ZhengMa is a very powerful IME, as it doesn't really distinguish between 'simplified' and 'traditional' so one can type both scripts without having to switch between modes. It's designed to be a system which can include all HanTzu漢字 used in China, Japan, Korea, Vietnam, so it's really convenient for people who enjoy reading old literature and want to type out phrases in Classical Chinese or in dialects.
(Yes, there are people on the Internet who actually learn and communicate in Classical Chinese. I found such people online - foreigners, not Chinese - (for example this room here https://discord.gg/Mn2Mcy3 ), and there are also blogs like this one https://kuiwon.wordpress.com/. On QQ groups also one can find Chinese people who are learning 古文 and 文言文. )
ZhengMa is not mainstream, and the reason for this is its power ;) Just like the LISP programming language is not mainstream. (and at the same time the 'mainstream' languages keep pilfering decades old features from LISP and present them as 'new' and 'cool' :D )
As someone in the 20th century said:
"My whole religion believes it; so many people cannot be wrong." This
argument I have heard so many times that I have made my own argument:
If there are so many people agreed on something, they MUST be wrong!
So many people cannot be right. Right belongs to very rare people:
crowds cannot be right, they cannot have the truth.
Here's the most comprehensive guide available: https://pan.baidu.com/s/1mgFsuA8 (ZM_PDF(2010).rar or 郑码实用手册2010.pdf) and really you don't need more teaching materials. Just get the basics then start practicing, it'll get into your bones and you'll love it and appreciate it more and more as you discover how much it helps you to learn and remember the Chinese characters.
(UPDATE: here are some oldschool tuts in EXE and XLS format which teach you ZhengMa and help you to practice it:
From that website you can also download the tutorials in swf format and other ZhengMa related tools )
Other useful links:
http://zhengma.911cha.com use it to display one or more characters' ZhengMa code
http://www.zdic.net an online dictionary that doesn't suck too much and shows the ZhengMa code along other useful info about the 漢字
John Cikoski in his Notes for a Lexicon of Classical Chinese said:
Reading Classical Chinese we rely heavily on character dictionaries
that vary in quality from poor to execrable.
for a good reason. Btw I recommend his book, it is VERY non-mainstream. Just reading the introduction can make one tremble.
https://zhengmashurufa.51240.com/ an online ZhengMa IME for people who are in some desperate circumstance and need to type something without having the IME installed
https://www.augsky.com/860.html more ZhengMa lessons & stuff
PS: Of course other IME's are great and useful too. Pinyin has its uses, handwriting IME's can sometimes be very _handy as well ;) It's best to combine all these tools and methods.
ZhengMa鄭碼 is one of the best tools available to help a Chinese student learn faster.