There's a divine command to preach, but I don't believe preaching is to be done just for its own sake. This is why I think it's vital for preachers (after prayerful preparation) to decide what they want their audience to do as a result of hearing the sermon or homily. This is different from deciding what one wants to say - whether or not one can summarise it in a sentence. Indeed, it's the desired outcome that might usefully be summarised. The more specific and measurable that goal is, the more likely it will be for its attainment (or otherwise) to be measured.
Once the objective has been indentified, there's no shortage of advice on how to preach well, not least in the form of books. The trouble is that such advice isn't consistent among all authors. Some approve of autobiographical preaching while others say it's distracting. Who is right? Preaching matters too much to be left to chance and/or considered a self-justifying art-form. If a homily sets out to evangelise, then it needs to do that in the most effective way possible.
Simply saying that there are varying styles of preaching, different types of congregation and several types of message doesn't mean that there aren't better and worse ways of creating and delivering a sermon or homily. There is a whole science of effective communication, one of whose findings is that we are more receptive to messages which are conveyed visually as well as verbally. Books on good preaching don't always cite research to prove that their advice works. In this way they resemble the multiplicity of books which tell us how to write and speak clearly.