Lumley's Mythos fictions runs hot and cold with me, and this collection was no exception.
The main meat, THE RETURN OF THE DEEP ONES is a short novel that feels like it could have been a lot shorter, given that the big reveal happens very early on and after that we're left with a lot of padding to make it up to a length.
BENEATH THE MOORS is better paced, and has a better sense of place, with its moorland and caverns setting being particularly well evoked. Again there's a bit of padding, in epistolary form, but the plot moves along quickly enough that it doesn't get in the way.
The other two tales are Titus Crow stories, one a nice origin story that was genuinely creepy, and the other the tale of Crow pitting his wits against an ancient sorceror that didn't do much for me at all.
So all in all, some hits, some misses, and nowhere near as imaginative and wild as his Necroscope work, but still, solid enough, and it passed the time well enough.