first off lets start with BY FAR the most important link.
- The Arreat Summit
http://classic.battle.net/diablo2exp/
this site with written by blizzard themselves and contains about 95% of all info you will ever need - ias calcs
Diablo 2 Attack Speed Calculator | ias calc (all classes and lod werewolf ias)
Diablo2 Resurrected 2.4 PTR2 Wereform IAS Calculator (2.4 werewolf ias) - Breakpoints
Status Tables for all Classes - immunites and others
https://www.theamazonbasin.com/wiki/index.php?title=Immune_to_Physical - how to read the map without maphack
D2 lod pathfinding - Google Sheets
https://nightfallx2.wixsite.com/d2speedruns/map-layouts - other good information
Sockets - Diablo Wiki
number of sockets per item level
Area Level - Diablo Wiki
area level
and lastly if u want to get into some really confusing stuff:
7. treasure class
it also might be handing to have a copy of 2.4 patch notes