Golly, I am gunna have my butt bitten saying this...
There is nothing you need to learn before you go. You will pick it up along the way.
What is best to study up on is the exciting stuff: The places you are going to! Their history, culture, geography etc. Then when you arrive you will be able to enjoy the new land and people even more.
Buy the book: Identifying Whales and Dolphins. Also theres Coral Reef Fishes (1 book does the whole world). Also have a look for a book of maritime birds - I dont have one and would like one.
If you are leaving from the USA East Coast the first big stop off is the Caribbean Islands. Being there a year is easy. Chandelrys are huge, boat workers excellent. That trip down from the US (via Bahamas or direct) is your shakedown cruise. That will give you the experience to know what upgrades you want on your boat. Do them in the Caribbean, not at home before you leave.
Some people spend years 'outfitting' their boat after purchase and then find they have totally bought the wrong stuff because the *think* they know... so wait until after your first long passages because then you *will* know
Other advice is not to read any books about shipwrecks, sail boat disasters, survival stories etc etc etc. They are usually all just scary stuff that hinders your mind instead of enhancing your mind.
A tropical downwind circumnavigation is a truly wonderful, easy, enjoyable non-scary experience.................... However, between now and when you go everyone will try to put the fear of .... into you. Don't listen to them!
Plan of action --> Buy Boat --> Get on board --> Sail South --> When you know the boat excellently then do your upgrades/projects --> Continue cruising at a slow pace forever!