The concentrations of 18 carbonyl compounds and their health risks for people were investigated in indoor and outdoor air of three different rooms of a library in Guangzhou, South China. Indoor air samples were collected during normal activities of staff and students, and carbonyl compounds were analyzed by high-performance liquid chromatography. The results show that the total concentrations of 18 carbonyls ranged from 8.8 to 73.9 µg m–3 with a mean value of 24.7 µg m–3. Acetone was the most abundant species with a mean value of 10.8 µg m–3 in indoor air, followed by formaldehyde and acetaldehyde. The levels of carbonyls in this study were relatively lower than most of other microenvironments in the literature. The average total concentration of carbonyls in the electronic reading room was higher than those in stack room and compact stack room. Diurnal variation in carbonyl concentrations was not distinct in the three rooms. The indoor/outdoor ratios and Spearman correlation coefficients demonstrated that endogenous emission sources contributed significantly to most of indoor carbonyls, while outdoor infiltration might be one of the main sources for acetaldehyde. The personal exposure, lifetime cancer risk, and non-cancer chronic health impacts through inhalation associated with formaldehyde and acetaldehyde were estimated and discussed.