What is the best way to book a room in the main Disneyland Paris hotel (Rue de la Marnière)?
The Hotel you are talking about is the Disneyland® Hotel.
The best option is to go directly on their website because they have a bunch of packages.
Buying from their website offers you a bunch of advantages:
What is included in a Disneyland® Paris vacation package?
Disneyland® Paris packages are one of the best ways to experience a carefree vacation filled with a magical mix of fun and convenience. These packages can include Travel, Hotels, Meal Plans, FASTPASS® and Extra Magic Hours, minimising your out-of-pocket expenses and removing the guesswork—but none of the fun—of planning your special time. Plus, all your package features are ready for you at check-in, saving you from waiting in multiple lines or having to remember to bring tickets from home.
Find out about Disneyland Paris vacation packages
PS: Sometimes they have good deals on a website specialised in private sales but you have to be in France to have access to it. I've just looked and at the moment they don't have a private sale. I'll keep looking and tell you if I spot a special sale or deal :)
When are you planning to go and I guess you are going with your kids (if yes, how many and how old are they?)