Only 'thief' webnovels I know about are RPG ones where they've got the thief class, but actually stealing is generally not a thing. Though lockpicking and disarming traps is common. Rebirth of the thief Abyss Domination I've read a hard cover book featuring a thief though, but I don't remembering him stealing much besides some paper money, that he for some reason doesn't realize is money, in the beginning of story: https://en.wikipedia.org/wiki/The_Redemption_of_Althalus