Here is a list of general resources for Abstract Games that I have used.
A resource for a particular game will be noted in the article for that game.
Game Playing Websites
Top of the list must be BoardSpace for PC, Mac, Linux, Android and IOS devices, run by Dave Dyer. This is where DrDave and I, as Alfiesboy, have been learning and playing games together online throughout the pandemic. There are no costs involved, just an online registration required. Most of the games are abstract, though there are also some commercial board games. I particularly like that for almost all of the many games there is a computer opponent to play with which greatly assists the learning process for new games. For some games there are 4 bots (Weak, Dumb, Smart & Best) and I prefer playing them to random opponents from around the world. Yes, I have been called unsociable.
The first general abstract games site I used was Zillions of Games. I discovered it after I retired, perhaps in 2015, whilst searching for info on Monster Chess. I loved it because it gave me a computer opponent to play against, no longer having school pupils as opponents. There is an underlying games engine that does all the game controls and tree search stuff, with an overlay of a rules file (.zrf) for each particular game. These zrf files are open for the user to inspect and modify. I edited a Phutball.zrf file to create a 1-D Phutball variant (articles coming soon). The Zillions demo comes as a free download with lots of games immediately available.

However, I purchased a key to unlock the full version for about £20. This gives access to a library of over 3000 zrf files to download, mostly abstract games coded by users, but some puzzles and miscellanea. These can be seen on the Zillions site, but I created a basic Zillions Game Listing.txt, which can be loaded into a spreadsheet or text editor. When introducing the site to DrDave, we discovered that this payment system for unlocking was no longer available, but it seems that it has now been replaced by a voluntary donation of an unspecified sum, though I have not tested this procedure. I really like the flexibility of the computer AI which shows the depth of tree search and a position evaluation as it 'thinks' - useful for analysis and can be set to think on a move forever!
I have recently started looking at igGameCenter There are several games of interest that are not implemented on Boardspace - Amazons, Camelot, Dots & Boxes, Epaminondas, Hex, Oware and Phutball. Since there are 148 games in all, no doubt DrDave and I will find others we like. On first impressions, it is not as friendly as Boardspace and it only has a computer opponent for a few games.
Even more recently I discovered Ai Ai. This is a similar site to Zillions of Games in that (if I understand correctly) there is an underlying gaming engine that does all the tree search stuff, with an overlay of the rules for a particular game that could be set up by a technical user. Not as friendly as Boardspace but looks as though it may have lots of sophistication - I need to have a good look. Perhaps more of a research site, but all games have PvP or computer opponents, so good to just play - oodles of games!
General Info Websites
Books
More websites, books, articles, suppliers . . . to follow (slowly . . .)