Let me start off with some caveats. I've met Sam Sykes. I like him and I'm going to read the next book in his Aeon's gate series. Sam is a young writer but a serious one, no matter what his twitter feed is like. He's also well-read in the Fantasy genre. He even has one of his characters, almost meta-texually, complain about "...now we're just sitting around in furs, talking instead of killing people."
So, that's a good review tag: Too much sitting around, not enough killing.
It's a touch trick. Sam Sykes clearly wanted to develop his characters more in this follow up to his action-packed debut, Tome of the Undergates. However it comes at the expense of plot and movement. Character revealed through action works better than sitting around and talking.
I will say this: if you can get through the first 120 pages, things do pick up.
When the action happens, it is intense and gory and sure to please fans of the first book. Sam also has a knack for some very beautiful and insightful prose. He's at his best when he's musing upon the philosophy of violence, the nature of faith and how emotional pain can be greater than any physical injury. Inventive monsters, races and an interesting world.
Ok, plot: After the events of Tome of the Undergates the protagonists find themselves adrift and lost with a titanic ass with a deathwish in the prow of the boat. The latter refers to the dragonman, Gariath. Gariath wants to die but apparently doesn't care to do it the easy way by just going over the side. Instead he attacks a sea monster with the broken shaft of the boat's mast, causing the boat to be destroyed. This separates the group and maroons them on an island populated with oversized cockroaches and a couple of races of lizard men. A great deal of angst ensues. Relationships are somewhat mended or complicated, the macguffin, the namesake Tome of the first book is read, lost, found and given to one of the books Big Bad Guys only to be lost in yet another ship's destruction. Attempts to quit adventuring, at least the mental intention to, and attempts at heroism are unrewarded, leaving the main protagonist, Lenk feeling betrayed. The titular Black Halo is briefly seen but is barely touched on. Also flying around the story is a Librarian, Bralston, who might be the most likable character of the series so far. However his contribution seems to be mostly a deus ex magica.
Gariath has the most complete character arc, seemingly coming out of his death wish, though it along with a number of complications are resolved a little too abruptly. The shict, Kataria,starts off with the apparent intention of killing her human companions, including Lenk, as a way of reconnecting to her heritage. This is a wonderful opportunity for tension that gets resolved way too early. Denaos comes off the best with his back history becoming more and more fascination. I'd much rather read a book with him as the main character. (In fact, I'd pay real money to read that book, Sam) Dreadaeleon begins to suffer the aftereffects of too much magic use in ways that are hilarious and painful. Asper has some angsty crises of faith and nearly gets raped. Lenk...well, Lenk gets a bit more back history that makes him sound a bit less crazy and he does reach out to Kataria but ends up getting betrayed and abandoned in a big way.