In my current project my customer has still got plenty of 2900s,4k,5k and 6500 switches running on CAT OS. For me, working with CAT OS is little hectic. The major reason being that it's kinda hard to find CAT OS documentation. Also no Cisco Networking course teaches CAT OS anyways so technically I was never been trained on this. But still customer has got plenty of those so I got to support these.
So I was looking for a consolidated reference/study material to learn CAT OS for quite some time and finally I found a nice book from Cisco Press:
The thing I very much liked about this book is that it has a parallel command reference for IOS vs Cat OS. Which makes it easy to find the right commands as if you know equivalent IOS command.