Sentence:  All I required in return for my services from the slave-dealer, whose brutal countenance and manners were truly repugnant, was to take me about the town, and explain to me certain mysteries in his craft, which knowledge might be useful in time to come. Little did he suspect who his interrogator was, and freely in his <WSD>unsuspiciousness</WSD> he entered upon the subject of slave hunting in the Somali country, and Zanzibar, of all things the most interesting to me. 
Meaning of the word marked with <WSD> tags:  The quality of being unsuspicious ("of" something); lack of suspicion; lack of awareness.