Life In 19x19

I am strugling with middle game kos, especially against stronger opponents.

Do you have some resources to suggest (youtube, books, forum posts)? Or any advice how to improve this aspect of my game?
I am interest in both general principles and tactical situations.

Thanks

There's probably a lot here, particularly by BillSpight, if you can find it.

https://senseis.xmp.net/?AllAboutKo is a decent book I hear, Bill gives it a decent review. Tony Atkins reviewing it says "The last book on Ko was published over ten years ago and was thought at the time to be the worst Go book around, a feat since overtaken by a book from Korea. However this book redresses the balance and is to Kiseido's usual high standards."
https://senseis.xmp.net/?FightingKoTheBook is the bad one Tony mentions.

John Fairbairn's books reviewing Go Seigen's games aren't focused on ko, but Go was a particularly good ko fighter even among pros so there'll be plenty of examples dotted about.

I wrote up on a book by Cho U which is available in Chinese and Japanese language.
https://tchan001.wordpress.com/2013/08/ ... -of-cho-u/