Polymorphism in molecular crystals presents a major challenge for controlling crystallization. Here, we combine experiment and theory to characterize the polymorph landscape of acridine and investigate its selective control by additives. Melt crystallization experiments resolve the previously unknown structure of form VIII, while topological crystal structure prediction recovers all seven known unsolvated polymorphs with determined crystal structures. Combining a fine-tuned machine-learned interatomic potential with anharmonic free energy calculations reproduces the experimentally observed stability relationships and reveals a shallow free energy landscape. Finally, experimentally observed additive-induced polymorph preferences correlate with mixed-crystal substitutional defect energies and topological similarities between additive and acridine crystal structures. Together, these results provide a comprehensive picture of acridine polymorphism, linking crystal structure, thermodynamic stability, and additive-controlled crystallization.