In 1990 and 1991 I read a collection of books about cycling from the local library, and I've picked up more books since (mainly about endurance cycling, training, bicycle mechanics, etc.) and have skimmed them. But very few of the books I own or have read have been about cycletouring, and I didn't use a book to learn how to tour.
Instead, I used experience ...
I have been travelling pretty much since birth, and cycling since I was 6 years old.
I've been doing hub-and-spoke style tours since I was in my teens. The bicycles came with us when my family went on holiday and we would cycle out this way one day, and out that way another day, and hike up a mountain another day, and do some more cycling another day ......
In my early 20s I got into camping, and more hub-and-spoke tours in conjunction with the camping.
I did my first point-to-point tour in my late 20s (1995) ... it was a supported tour down the Icefield Parkway. And then I got into racing, and then randonneuring, and more hub-and-spoke and short overnight tours, and lots of cycling in general. My first unsupported point-to-point tour was in 2003 through Wales.
Ride lots - get to know your bicycle and your body. You'll discover things that don't work for you on your bicycle, and you'll want to make changes (this is where a bicycle mechanics book might come in handy). You'll discover, as you increase your distance, that perhaps there are things that aren't working for your body (this is where a good training and nutrition book might come in handy).
Ride a variety of places. Get a map, pick a spot and ride there. Ride hills. Ride in wind. Ride to local tourist attractions. Become familiar with all the roads in a 50 km or 100 km radius of where you live.
Go camping, if camping is your preference for accommodation. Buy or borrow some camping gear, load it into the car and go camping for a weekend or two. You'll soon discover that this is working well, but that is not.
Travel lots - take every opportunity presented to you to go new places, whether the new place is the next town over, or the other side of the world. Go!
And then start putting it all together and start touring.
That said, I do own two books about touring and cycletouring, although neither one is a "how to" manual. Both were somewhat motivational/inspirational.