system
A chat on 4chan
human
by far the dhammapadha. it seems simple and uninteresting to the unexperienced. but as far as i can tell it has around 4ish layers to it, and as you grow as a person and reread, you will sometimes find a whole new dimension to it and 'realize' (in the original sense of the word) some deeper truth than what is just written. I busted a detective nut when I saw the second layer (despite already enjoying the first layer), and when I saw the third I considered dropping everything to go live innawoods. I am now in the process of memorizing it verse by verse. 
Secondly I'd say the Tao. It parallels the dhammapadha in many ways, but the wording is very very different, but if youve seen the second/third layer of the dhammapadha you can see the tao for what it truly is despite the translation and even orginal wording being somewhat rough to understand.
Hermetic principles is pretty good too, but its also very direct (maybe thats preferable to some), so theres no unlayering and what not.
gpt
Like the Tao te king or liber AL
human
Look at me I didnt read you whole post before replying; you mentioned the tao te king. I guess that means I know about them deeper understandings and sheeiit