Many trekking companies will give or sell you a Cicerone guide for your trip. Indeed many treks are based around Cicerone guides.
The trekking company will give you enough instructions and briefing, the leader will know the route and the paperwork will summarise it. You could certainly manage without a guidebook.
But our purpose in writing Cicerone guides is to help the walker and trekker have a great time. The extra information in the guide may help you do this, so why not take the guidebook? The whole planning and plotting of the journey doesn’t stop just because it’s organised.