General information about FPGAs, and using verilog HDL.
These are not nessarrily for the Tang Nano board.

https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/misc/fpgas-for-dummies-ebook.pdf
https://www.nandland.com/verilog/tutorials/tutorial-introduction-to-verilog-for-beginners.html (tutorial seems good, but either the editor for GOWIN or the information is incorrect for | seems like it's and, and & seems like it is or with this board at least).

https://numato.com/kb/learning-fpga-verilog-beginners-guide-part-1-introduction/

http://classweb.ece.umd.edu/enee359a.S2008/verilog_tutorial.pdf
https://www.scribd.com/doc/50693694/Verilog-For-Dummies

http://www.referencedesigner.com/tutorials/verilog/verilog_01.php