Marrakesh

He offers Lonely Plant's guide here on the website and doesn't appear to have published his own RS version:

https://store.ricksteves.com/shop/guidebooks#spain-portugal-morocco

Does Rick Steve's have a Guide Book on Morocco and more specifically, Marrakesh?

Nope. Personally, I found the Cadogan (popular series published in the UK) guidebook on Morocco to be excellent.

Take the train from Casablanca going south...

Great song, but not necessarily good travel advice. Skip Casablanca. Skip all the coastal ferry ports and nearby towns that are easy to get to. Instead, fly direct to Marrakech from someplace in western Europe on a low cost airline, it's super cheap and unless you have a month to spend, I'd suggest there's nothing on/near the coast worth stopping for - the good stuff starts at Marrakech and Fes and beyond, might as well go straight there and bypass the day-trip/bordertown experience of the ferry ports.

Fodor's also has a guide for Morocco.

I was in Marrakech for a week many years ago. I am sure much has changed but some things will be the same:

Have a beer on one of the rooftop cafes overlooking the J'maa alFna, visit the Mamounia Hotel and sit in the gardens where Churchill painted during WWII as a way to get away from the pressures of the war, and when you get tired of all the "salesmen" (the kids that harangue you to buy stuff every minute you are there) Start using sign language with them they just give up and go away...

We just left Marrakesh yesterday and used ''DK's top ten Marrakesh" while there. It hit all the high spots. Finished there on A Friendly Planet tour.