Get the book "stepmonster"

I think "stepmonster" is great, but I don't know how well it would work given that your SKs are adults.

Honestly, I think this forum is your best resource, because people here have actually lived it and still are. Many of these "experts" aren't stepparents and haven't actually lived it.

I think Stepmonster will validate your experience as a SM. My skids are adults and it helped me enormously to read it. It put everything into perspective for me.

"Step Wars" or more recently "Making Adult Step Families Work" by Grace Gabe and Jean Lipman-Blumen is a good book. "Step Wars" was published first and "Making Adult Step Families Work" is the same book in paperback. It walks you through various scenarios from the adult kids' perspectives and from the parents' perspectives and then proposes some alternative approaches. "Stepmonster" was great but limited for adult stepkids and those who lost a parent to death rather than divorce.

This may seem totally unrelated, but it actually is.....

"Codependent No More" by Melody Beattie

It will help you have an understanding of what and who you are dealing with.
There is always some form of codependency in families - some more than others.