Harlequins are now a separate army, so depending how many you have you may want to just skip for now (you'd also need a copy of a white dwarf with their new rules update too).
The Eldar codex is 8th edition and will be replaced, but that doesn't look to be soon.
If you have the
40K app, all of the 8th rules are free so you could bypass all these books altogether and just access with the app.
Phoenix Rising added custom Craftworld traits, Exarch powers for Aspect Warriors and a new psychic tree.
It also has rules for Ynnari, which is a brand new Eldar faction (they worship the Eldar God of the dead). The Ynnari are a mix of Eldar, harlequins, and dark eldar. The rules aren't great at the moment and require the use of special characters that are bought together in a set (the Yncarne is quite a cool model though)
https://www.games-workshop.com/en-US/Ynnari-Triumvirate-Of-Ynnead-2017.
The good news for you is that your old models are still absolutely usable in 9th, whether you can put together a strong list is a different story. 9th is all about objectives...primary take and hold type (across the table) and secondary ones that you choose from a set (based on killing enemy, seizing territory, performing "actions" etc). The downside for Eldar is that they lack resilient troops to hold objectives, so mechanized Eldar seems to be the way. You try to remove enemy from their objectives while parking transports with embarked troops on yours. The exception is wraithguard (the axe/shield version which you probably don't have) and shining spears who are tanky and killy.
We are all waiting for a new codex and there is a current discussion about how that might work in this very lethal new 9th edition.
Welcome back.