The given sentence is in present perfect tense which is signified by the words 'has' and 'kept' (past participle form of keep).
So, in passive voice it will be- Where have been the books kept by him? (has/have+past participle form of the verb).
Kept is the third form of the verb keep and we use have as books are plural
The action is being acted upon 'him'. Thus it becomes passive.