September is months away. You've finished Heat and you need something to fill the void. These books share specific DNA with Penn Cole's series, whether it's the slow-burn political romance, the mortal-among-immortals dynamic, or the heroine who refuses to stay quiet.
Fourth Wing puts you in a war college where the stakes are lethal and the romance is enemies-to-lovers with real teeth. Violet is smaller and more fragile than her peers, which forces her to rely on intelligence over brute force. Sound familiar? The dragon bonding adds a layer that Everflame doesn't have, but the core dynamic of a woman proving herself in a hostile environment while falling for someone she shouldn't trust hits the same notes.
The Serpent and the Wings of Night by Carissa Broadbent is probably the closest comp. A human girl raised by a vampire enters a deadly competition surrounded by creatures who should want her dead. The romance between Oraya and Raihn builds through forced proximity and mutual danger. The spice level matches Everflame's later books. The emotional devastation matches too.
An Ember in the Ashes by Sabaa Tahir takes the power imbalance between mortals and a ruling class and makes it the engine of the entire story. Laia is at the bottom. She has to work her way up through a system designed to crush her. The romance is forbidden and builds slowly across four books. If Diem's refusal to accept injustice is what hooked you, Laia's arc will do the same thing.
From Blood and Ash by Jennifer L. Armentrout gives you a sheltered protagonist who discovers her entire life has been a lie. The romance escalates from forbidden to consuming. The world-building reveals itself in layers. The twist at the end of book one will rewire your assumptions, which is exactly what Cole does in Spark.
A Fate of Wrath and Flame by K.A. Tucker features a woman thrown into a kingdom where she's supposed to be someone else. The political intrigue is sharp, the romance develops against a backdrop of genuine danger, and the heroine has to figure out who to trust in a world where everyone has an agenda. The pacing matches Everflame's middle books, where every chapter adds a complication.
The Bridge Kingdom by Danielle L. Jensen is shorter and punchier but shares the same core tension. Two people on opposite sides of a political conflict, forced together, falling for each other against their own best interests. Jensen writes political romance with precision. Every scene earns its place.