It is, of course, correct. This book has sold millions upon millions of copies over the last 80 years. If there had been a grammatical error in it , I think someone might have noticed!
Bear in mind that the word 'for' is not a preposition in this sentence. It is not 'for' in the sense of 'for me', requiring an object pronoun. 'For' is a conjunction here, meaning 'as' or 'because'. It's a link between the two parts of the sentence. The subject of the next sentence is the pronoun 'I'. The word 'myself' simply adds emphasis. The sentence is fine.