Firstly, I am not sure I would agree fully about 'continue' being the meaning of the present perfect. The perfect denotes some sort of completion as a primary meaning
IMO. Putting that to one side,
I have continued not to buy books = zero books bought. I have not continued to buy books = zero books bought. There is a slight difference, but I think that would require the broader context to know whether the person has not bothered to buy or has had a policy of not buying. From the sentence, we don't know what came before and don't know if the person is saying it as they come out of a bookshop with a carrier bag ful of novels or whether they are explaining why they cannot even come inside a bookshop with me.
I think that 'not to do' and 'to not do' are different- the second is a stronger refusal to me. However, some will not make such a distinction, so an assumption that it is universally stronger is inaccurate; the context defines so much of themeaning. If I say 'I have lived in London.', it is impossible to say whether I live there now or whether I am describing a past experience that is relevant to the conversation. Tenses and aspects can be used in many different ways, and it is often the context that determines this.
Soemtimes there is a clear grammatical structure controlling changes in meaning; think of the difference between 'He was going to go to the party' (= in all likelihood he didn't go) and 'If I'd know he was going to go to the party' (= his surprise presence irritated me), but I see nothing in the sentence to pin down the motive behind the person's booklessness.
We just know the period of no book purchases, not whether it has ended or not, but that there is some reason for the person to bring it up, though we don't know the reason. It could be poverty, idiocy or a boycott., the reasons could be external. I don't think there's enough information here to carve the living beating heart out of this 'haven't bought'.