Yeah, a bunch of people had an "expanded Lovecraft universe." Take the tone and theme and put your own spin on it. You could argue that King's It is Lovecraft influenced.
Derleth and Bloch mostly did short story work in the Lovecraft universe, so there is no signature work I can pick out. Or my memory is burnt out. Either is equally likely, lol.
For Derleth, I've read most of his work in various anthology collections (most of his stuff went to Weird Tales. Here is an anthology of his own work. "Shuttered House," "Paneled Room," and "Return of Hastur" are the ones I remember.
The interesting thing was Derleth started Arkham House publishing mainly for the purpose of publishing Lovecraft's work. He also introduced the phrase Cthulhu Mythos.
Lovecraft was a direct mentor to Bloch (Haunter of the Dark was dedicated to Bloch). I have some version of this book by Bloch.
Hope that helps some.