Mouseferatu said:
Actually, they're not, although they have the same author. Check the descriptions.
It's the description that actually made me think they are the same... I thought that Exemplar of Evil was the first draft:
"a book about creating BBEG of different levels, with 9 examples including stats, adv seeds, camp hooks, minions and lair"
And that later the development had resulted in more focus on the ready examples than the creation guidelines, and also in mostly high-level BBEG, hence:
"a book about creating BBEG of different
high levels, with
9 examples including stats, adv seeds, camp hooks, minions and lair"
They're both hardcover, same page count, same price, same author. The second is due only 3 months before (a delay of the early schedule?). One of them doesn't have the cover...
At least someone could suspect

it's strange for WotC to publish two similar books within 3 months only... But you're an insider so you know better what's going on there.
Elder Evils said:
Elder Evils provides DMs with truly wicked threats to challenge high-level heroes. Each "elder evil" described in this nightmarish tome comes with a detailed description, stat blocks for the elder evil and its minions, tips for how to incorporate the elder evil into any D&D campaign, adventure seeds, and maps of the elder evil's lair (complete with encounters and room descriptions). The book also provides guidelines to help DMs create their own unique elder evils as worthy campaign villains and endgame encounters.
Exemplars of Evil said:
The Exemplars of Evil supplement shows Dungeon Masters how to construct memorable campaign villains and presents nine ready-to-play villains of various levels that can be easily incorporated into any D&D campaign. Each villainous entry provides complete statistics for the villain (or villains), as well as adventure seeds, campaign hooks, pregenerated minions, and a fully detailed lair.