Reddit mentions of Stephen King's The Dark Tower Concordance

https://www.etsy.com/market/stephen_king

Has plenty of gift ideas.

This book is good for fans as well:

http://www.amazon.com/Stephen-Kings-The-Dark-Tower/dp/1451694873/ref=pd_sim_14_4?ie=UTF8&dpID=51P2JRHHcSL&dpSrc=sims&preST=_AC_UL160_SR107%2C160_&refRID=0C6XDDCQBNJ1XMHJ9GWA

Totally worth it. It's a must have for any big fan. It adds all kinds of extra content that wasn't in the books. It's not written by King, but fully endorsed by him. It was developed in part by the work of Robin Furth, King's personal research assistant. Apparently when King started writing the last three DT books, he hired Robin to research the first four and write the concordance. Robin and some other people produced the comics which include stories referenced or not included in the books. The wind through the keyhole was actually dedicated to her and the people at Marvel. Also, the artwork is amazing.

I bought this book for my boyfriend because we love the series. It may help you.

http://www.amazon.com/Stephen-Kings-The-Dark-Tower/dp/1451694873

His characters and settings appear interwoven throughout his books. There are a large number of them that relate to the Dark Tower series, in either direction (the book makes reference to the Tower, or the Tower series makes reference to the book). The world of The Stand, for example, makes a pretty heavy footprint on one of the DT books. I don't have time to do a complete concordance (and it would fill a book if I did), so suffice it to say this kind of thing is EVERYWHERE in his books.

It's already out.

I'd recommend both of these books:

https://www.amazon.com/Stephen-Kings-Dark-Tower-Concordance/dp/1451694873

https://www.amazon.com/Road-Dark-Tower-Exploring-Stephen/dp/0451213041/

The concordance /u/spewner got me is awesome. It's really in depth. Tells you the page numbers and everything. I would highly recommend it. I'm loving it so far, not many refrences to his other books so far, but I'll be patient :)

http://www.amazon.ca/Stephen-Kings-The-Dark-Tower/dp/1451694873

I've read about 90% of King's work.

If you just want a quick rundown on King's universe without reading an entire library full of books, someone made a pretty handy guidebook for reference:

https://www.amazon.com/Stephen-Kings-Dark-Tower-Concordance/dp/1451694873

WARNING! This books is pretty much nothing BUT spoilers, so if you think you might read any of his books in the future you're probably better off avoiding this.

Also checkout:

The Road to the Dark Tower: Exploring Stephen King's Magnum Opus

And

Stephen King's The Dark Tower Concordance