The author wants to attract people to read his book. For example, someone browsing the book at a bookshop will often open the book and read the first few pages or the author's comments at the beginning. The author is hoping that if he writes this introduction himself then the introduction will 'read better'/seem better/sound better to the reader than if someone else wrote it, and that reader will then go on to buy it.

Hi there, good question! 'in favour of' means to be in support of something. The first clause can be understood as 'the writer wants to support the reading of his book.' The dependent clause of this sentence is 'by writing the preface' So, he giving additional information that will support the overall understanding of his book.