I am trying to create a kind of book of those that are made with letter size sheets folded in half lengthwise. On my own I tried several things but I only managed to get 2 letter pages joined vertically and that is not what I am looking for. It is important to clarify that the pages will be printed on both sides. In my country we call it booklet ("folleto" in the image). I was working in Publisher but I saw that inDesign offered me better things, and this is the configuration I was using.
To create a "booklet" with a final size of 5.5"x8.5" (Letter half) you want the InDesign pages to be 13.97cm x 21.59cm (5.5"x8.5").
When 2 pages are next to each other they will create 1, letter-sized, horizontal, spread.
Upon output you can choose to output single pages (5.5x8.5") or Page Spreads (11x8.5").
Most often a commercial printer will want single pages so they can properly impose for a booklet. If you are doing this yourself at home, you may need page spreads output so you can manually bind the booklet.