Generally, if you are comfortable, then your books are probably in a safe environment. Below 70 degrees Fahrenheit and below 50% Relative Humidity is a good benchmark.
Your key goal is a stable environment where mold is unlikely to thrive. Mold is one of the most difficult and expensive issues to trreat in a library collection. MAintaining a mold-inhibiting environment is one of the most important things you can do to preserve your library collections.
