The answer to the question above would be that the most effective way to combine sentences 8 and 9 would be the following one: A feature of Poor Richard's was advice about how to be happy and successful.
The result of this combination is a sentence which is pleasant to the ears, it does not sound odd such as the first and the last example. The first example is too choppy because of the commas and the third one has a really strange structure which is not idiomatic at all.