The Ehrich Weisz Chronicles by Marty Chan

Set in 1890s New York, this steampunk fantasy pits Ehrich Weisz and his mentor, Nikola Tesla, against creatures from other dimensions. They join forces with a group of rebels to help stop a full-scale invasion, but the ragtag group might not be a match for a warlord and his exoskeletons.

Books in the Ehrich Weisz Chronicles:

Ehrich Weisz Chronicles: Demon Gate
Ehrich Weisz Chronicles: Infinity Coil