Fungal biodeterioration is among the most persistent risks to paper-based heritage in libraries. Cellulose-degrading and xerophilic molds—including species of Aspergillus, Penicillium, Cladosporium, Rhizopus, Mucor, Chaetomium, and Trichoderma—discolor, stain, weaken, and ultimately fragment paper and bindings. This review summarizes (i) the main fungal taxa implicated in library settings, (ii) environmental drivers and their control, (iii) current detection and risk-assessment methods (culture-dependent, ATP bioluminescence, and molecular surveys), and (iv) evidence-based preventive strategies aligned with ISO 11799:2024 storage requirements. We highlight the growing role of rapid and nondestructive diagnostics and holistic indoor air monitoring for early warning and risk management.