Titus Crow: The Burrowers Beneath, the Transition of Titus Crow by Brian Lumley
Your mileage on this depends very much on how much you like Lovecraft, and how much of a purist you are.
The Burrowers Beneath is a Lovecraftian pastiche, with a scientific veneer. An epistolary novel, I find it disjointed and stilted at first, but a couple chapters in I am rolling with it, and quite enjoying things.
These are not toward the Horror end of the spectrum, there is much pseudo-science, and both novels are more adventure novels, the second leaning heavily toward a Dreamlands style.
The Transition of Titus Crow is bat-shit crazy, and a lot of fun, I found.
I like Lumley, and you can see the seeds of a lot of his later work here, and he does get much, much better when he creates his own worlds. But he’s fun here. He loves the stuff, I love the stuff, presumably you love the stuff if you’re reading this…if so, you might get a kick out of this. You might not, as well, depending on what it is specifically you like about the Cthulhu Mythos. Me, I like many different flavours, and this volume is a hot gumbo of different Mythos styles, and I read it for the most part gleefully.