Hour of the Dragon
will always have a special place in the Howard library due to it being the sole Conan novel, but even taking that into account it's a fantastic read. One would think that Howard would struggle with a longer format having seen his vast collection of short stories, but HotD
suffers little bloating, dragging or padding, and is a very successful transition from short to long form for Conan.
Although there are many cannibalised elements from other stories, given the circumstances around its writing I think it's perfectly reasonable, and it isn't as if history doesn't repeat itself in real life anyway.
Although not the last written, I think it works as a fitting finale to Conan's career: it's as if all his other adventures were leading up to this one epic journey, the trials of Phoenix
being a rehearsal for his greatest challenge, where he'd have to employ all his skills and experience as a thief, mercenary, captain, general and king to save his kingdom. All very hero's journey stuff, but no less fun and exciting for it.
Considering Howard's impressive villains up to now, he really outdid himself with Xaltotun. He's probably the most awesome (in the original sense) of Howard's sorcerers: even heavyweights like Tsotha-Lanti and the Master of Yimsha seem like small fry against Xaltotun's ancient evil. Orastes' revelations in chapter 20 do a lot to hit home just how dangerous and powerful a threat he is to not just Aquilonia, but the world. We see other wizards and gods use magic similar to Xaltotun: Khosatral Khel recreates Dagonia as Xaltotun planned, Tsotha-Lanti incapacitates Conan with ease, Thugra Khotan was an ancient sorcerer resurrected after thousands of years. By combining elements of other sorcerers, Howard makes Xaltotun seem that much bigger and more impressive by hearkening back to them, whether he meant it or not.
So much else to say, but I'll be brief: A lot of people found Conan's detours with the ghouls in Argos and Akivasha to slow the story down, but I liked them.