E is for Eldritch

Eldritch

I love that word.  So much cooler than "magic" or even "arcane".  Eldritch implies something old, even occult.  If you put the word Eldritch in your book's title, chances are not just good I'll look at it, it is nearly a guarantee.

Eldritch Wizardry

This is one of the first books I ever got for OD&D.  It took me a bit to realize that OD&D was essentially the same as Holmes Basic D&D (I had not gotten my 3 LBBs yet).  But I loved this book all the same.  It introduced Druids (which I played like a witch), psionics and demons to D&D.  I had of course already seen all of these in AD&D, but to have this little book was like finding some lost, eldritch even, treasure.
Of course I could never buy this one near me.  The cover prevented that, heck even the game store I ordered it from (which would late go on to become my Favorite Local Game Store) even kept it behind their counter.  Thankfully I had parents that didn't mind me spending my hard earned money on this stuff and I think I paid 18.00 for my old copy.  Which at the time seems insane for such a small book! (1985).
Today, good copies are much more expensive and even fair quality ones go for more than twice what I paid.
Despite the fact that the "D&D" I play now is fairly far removed from this book, and other books have super-ceded and surpassed it, I still like to pull it out every so often and remind myself of the wonder it used to bring me and how can I capture that same feeling for others. Plus I am convinced that this cover inspired this newer picture of Demogorgon (who was introduced in this book) from Hordes of the Abyss.

Eldritch Secrets Vol 1.
This extremely attractive book is full of all sorts of new spells, feats and magic items for all the spell-casting classes in Pathfinder (or the 3.x product of your choice).
The vast majority of this book is devoted to spells and there a spells here for every need and situation.  There a few new feats and some magic items, but the spells here are the main course.
There are several useful Appendices including Spells by Descriptor, School and use.

At 12 bucks this product is not exactly a steal, but really, really close.  The art and layout is fantastic and you get a printer friendly version as well.

Eldritch Witchery

This is the only book in my list here I have not bought.  That is because I wrote it!
It is due out from Elf Lair Games sometime (it's still in editing and layout).

With this book I wanted to capture everything eldritch about the witch class.  So unlike some of my other books, this book also has a Warlock class.  It is also different from my Basic D&D book "The Witch".  Though all three classes together would make a nice full picture of the witch as she has appeared through out the history of RPGs.
Plus this book is designed to be used with the Spellcraft & Swordplay game.  It *can* be used with OD&D/S&W, but you would need to modify somethings a touch.
Obviously this is my homage to Eldritch Wizardry, though there is plenty more to my EW. No psionics.

Other "Eldritch" Games 
I also bought these games based on the title alone.

Monte Cook's Book of Eldritch Might and The Complete Book of Eldritch Might.
The definitive sources for magic in the 3.x game.  I have these, but never really used them very much.  They were great reads though.

Eldritch Role-Playing System
I agree with what some others have said, cool concept, cool game, bad layout.  Still though.  There is some very cool ideas here. I enjoyed it and can mine it for ideas for other games.

Eldritch Ass Kicking
This is a rather cool game of magical battles.  I got it when it first came out, but forgot about it.  I looks like a very fun game or maybe even ideas for a sub-game in a larger magical based one.  I do like how the skills were used and how it is expected that wizards do everything with magic.  So this not a magic-lite game!
I also love the art work, but I have been a fan of Thomas Denmark's art for a long time.
There are even some add-ons for the game.