I'm currently reading the Dark Tower series in editions which are published in 2003/2006/2012 (Specifically these). They all (1–5 at least) feature the same introduction, written by King on January 25, 2003; beginning:
Hobbits were big when I was nineteen (a number of some import in the stories you are about to read).
I tried to keep that in mind while reading the book 1: The Gunslinger, but the idea faded quickly as I didn't stumble onto 19-references. Through books 2–5 I noted the whole introduction chapter was same as in book 1, while the following "Foreword" or "Argument" was more like an introduction to the book in question.
And hence I disregarded "in the stories you are about to read" as a reference to the series and thought of it more as a reference to the stories in the introduction (King writes about being 19 years old, and how we was at 19, and how 19 is an important age, and how he found out that "Pride goeth before a fall" at around the age of "19 × 2" etc.).
Fast-forward to book 5: Wolves of the Calla, which I'm reading right now at about 10%, the references to number 19 are rather in-your-face and I immediately recalled and reread the introduction chapter… which raised my question:
Did I miss references to number 19 in the books 1–4? Or is it more or less happenstance the introduction mentions the importance of number 19—because Wolves of Calla was first published on 2003-11-04 and the introduction was written on 2003-01-25, so King was probably working on WoC on some level while writing the introduction to the series.
I'm not asking about what the number 19 could mean or how it is referenced in books 5–7/8; I don't want to spoil the fun. The reason I ask this is because the introduction hints it is important, but I haven't experienced that until book 5.