[r3n]

Official guidebook for basic knowledge http://www.nethack.org/v361/Guidebook.html

For more advanced mechanics & spoiler http://www.steelypips.org/nethack/

There is also a NetHack wiki https://nethackwiki.com/wiki/Main_Page

[r3n]

Public servers like NAO is also a good place to learn NetHack. https://www.alt.org/nethack/

You can download session files from players and play it on your machine, their configs are also available on the website.