"Horace Danby was good and respectable - but not completely honest". Why do you think this description is apt for Horace? Why can't he be categorised as a typical thief?
The author describes Horace Danby, a fifty year old man, as a good and respectable man in the society who earned well by making locks. He was successful enough to afford two helpers.
However, his riches were not sufficient to fulfil his desire of having rare and expensive books, which was why he stole once a year.
He can't be categorised as a typical thief because he was not a professional who earned his bread by robbing people. He did not harm anyone and neither took any weapon ever. He used to steal some money once a year and purchase rare and expensive books with the stolen money.