Let's start on signs, simple wooden sign, limited to 3 lines, when you place it, you can write message, a pop up window will appear, simple text box over a wood sign image (no post) and you will be able to write, after when a player clicks it they can view the words... Each sign will probably be around 5kb, very low on space, we want signs! :P
With signs being done, it can be the basic building block for all future items a player can write text on, so now books... Each page will be limited to fit text, maybe 30 characters maximum, you can't erase the writing, but you can destroy the book, to write you need to craft a quill from one feather, and ink maybe made from dye? That or add ink sacks or something. You'll then need to make a book from any material you think is realistic, maybe sugar cane and water? Book will have 3-5 pages and you will need to make it like a sign, place it as block, hit it with quill in hand, if the book's full of words (90-150 characters) you will need to make another, if not it will open up. Word count will be at the top right and also a x button to exit.
Feel free to add on to the suggestion. Thanks for reading!