Def the best WOTC purchase I have made is the Secrets of Xen'Drik book for Eberron. A 160p hardcover that retails for $29.95 MSRP. I got it on a closeout for $5.00 BRAND NEW. For those who are not familiar with Xen'Drik, it is Eberron's "lost continent", where 10s of thousands of years ago Giants (Titans? it's intentionally left vague) ruled, and the first Elves were a slave race. Ancient Xen'drik had an advanced level of magic and society that modern day Eberron has not been able to approach. Ultimately the Giants/Titans brought about their own destruction 40K years ago and were plunged into barbarism-they have never recovered. Xen'drik is packed full of "the unknown" as the entire continent is basically unexplored. Only two major towns exist, and one is basically "off limits" to the PC's anyway. Another cool feature of Xen'drik is The Travellers Curse. There are still areas where the ancient magical destruction has ripped through time and space, and basically a journey that might take 3 days going may take 2 hours, or 6 months coming back To quote the book "the Traveller's Curse twists time and distance, both in perception and reality". There are also random "shifting zones". What may have been an arid desert plain yesterday could be a volcanic field or arctic tundra the next. These of course are things which have made Xen'drik dangerous to explore and hard to map out with any certainty. Much of what is known about the continent's History and the land itself is based on legend and conjecture, not always fact. All of this this (at least for me) makes Xen'drik a DM's playground (and it is designed as such)
The book text itself is filled to the brim with plot hooks (both readily apparent and between the lines), and specifically there are several "mini adventure locations" meant to be placed anywhere you like (e.g. The Ancient Harbor, Old Growth Ruins, The Forsaken City, etc), a 100 entry list of adventure hooks, encounter trap areas (think Indiana Jones), several 2-3 page "adventure frameworks" which add enough detail and direction for the DM to build upon (these include basic plot and theme, suggested sites, PC level and encounters/EL,etc.). There are also sections devoted to magical and psionic locations (e.g. Memorial Marker of the Elven Dead, The Timeless Fountain, etc.), curses, magical item effects (not crunch, but flavor) and forgotten civilizations. I find all of this material sufficiently "generic" to be used in most any D&D campaign. And thats why it has been such a great buy- I can drop nearly all of this into any campaign I'd like since much of it's is not tied to Eberron proper, or names/power groups, etc are easily exchanged for ones approriate to your campaign.
Of course like any WOTC book there are new monsters, magic items, feats, PrClasses, equipment, encounters/NPC groups, etc. A couple of the monsters are very Eberron centric, but the majority are usable for other worlds with no problem and include sample encounters for each monster type. There are also plug and play encounters consisting of things like a "drow scouting party" or sahuagin raiders" -These are designed with the Adventure frameworks in mind, but are just as useable for your world and adventures (side note- the Xen'drik's take on the drow is the best thing since their original appearance in the G series- none of that FR drow angst/Elminster's girlfriend crap). There are some neat artifacts and artifact spells ( "always inscribed on some large immovable object, such as an immense stone wall, or a towering crystal pillar"-these can be cast once per year if the runes are deciphered, however the caster must again find the place/object where discovered to re-learn it- they are too complex to translate into a written form, etc).
Get's a two thumbs up from me. I would def reccomend to take a half hour or so and read through some of it at your FLGS or Bookstore. I passed over this several times at full price- I really liked what I had read, but $30.00 for 160 pages is a bit steep IMO. However, now that I'm very familiar with it, I would have paid full price for it. It's not long on pages, but is def long on utility. Hell, it may even convince me to run a Xen'drik centered Eberron game.