1. The CBOE site is a good place to start.
2. I picked up what I know through experience of trading it and researching it at the time on how it works. In 2016 and 2017 I sold Iron Condors on every SPX expiration available at the close prior to expiration date. I thought I had a "Cash Cow" that would last forever. However in 2018 the market makers could just not get enough premium into the options and I was getting too many max losses so I quit. Price just blew right through my IC protection.
I don't know of any websites that specialize in the SPX but I am sure there of some out there.
I will give you a couple of caveats though.
1. Stay away from the Monthly AM settlement. That is crazy and I don't know why they even have it. Trade only the Weekly's. Strange name in that there 5 expirations in a week now.
2. There is no underlying ETF behind SPX like there is in SPY so you can let it settle without getting out of your option at the close. It is a cash settlement into your account. It is much easier that trying to exit a position near the close. The bid gets below intrinsic value so you can really get hosed if you are not careful in a late close out. I let all my Iron Condors expire on the cash settlement process and it is smooth and seamless. Statements sure look strange as they show full value of your settlement contracts.
3. The great thing about SPX is that profits are taxed like a futures contract. A lot of wheeling and dealing with congress to get that done. So, 60% of an SPX option trade profit is taxed as a long term gain and 40% is taxed at ordinary rates.
4. Trading SPX options is a lot like trading TSLA options. There is so much premium early in expiration day that you have to have movement or the option just melts away. The last hour prior to expiration has been fun on TSLA the last two weeks.
5. I went back on TOS and took some screen shots of Friday's action of the SPX option chain close to the turning points during the day that shows how you can use it for good profits "if you have an edge on determining direction". I could post those if you or anyone else is interested.
Yes, post it.