The first "adult" book that I ever read was Stephen King's "The Stand" and I have been hooked on King since then though I have started venturing out to Dean Koontz (which I actually prefer these days - the Odd Thomas series is excellent). I am currently in the middle of King's "Cell" which so far is very good and much more like his earlier work.
As for the Dark Tower series, I was never able to get into it. I read up to somewhere at the beginning of "Wizard and Glass" - not sure which book in the series this is - and have not gotten any further.
I can relate to the sad feelings when a series is over. I am still morning the end of the Harry Potter series.