The historian was reluctant to disclose the contents of the newly discovered letters.

disclose 由前缀 dis-（表示"否定、相反"）和 close（"关闭"）组成，字面意思是"不关闭"，引申为"公开、透露"的意思。这个词强调主动将原本隐藏或保密的信息公开出来。