Read: Map of Bones
A fast paced story, a plot twist that is well planned and despite its development is not obvious, and a set of characters that (even though they are far from being fully fleshed out) indeed has some characters that are not perfectly exchangeable with standard cut-outs make a good action adventure, entertainment. Map of Bones has all this.
Compared to the first Sigma Force novel, Rollins has substantially improved the character development. There are no strange coincidences piling up on each other that are needed to cover up plot holes. Plot development has improved as well. Hence, even though the story is no less “fantastic” this time it seems much more credible.
I am not sure the (modest) romance is needed. Yet, it also does not distract. In the end, do not all action thriller have a little romance so the hero has someone to rescue, to prove his valor, and to be attracted by?