Sounds like his philosophy fit may have fit in with the broader prosperity gospel space, Napoleon Hill, Billy Graham, Norman Vincent Peale, et al. Potentially worth looking into. Also related to the self-help movements and the New Thought philosophies.

fascinating that he wrote a book Copywriting and Direct Marketing. This may also tie him into the theses of Kevin Phillips' American Theocracy?

Link to: https://hyp.is/E4I_qgvCEe2rQO9iXvaTgA/www.goodreads.com/author/show/257221.Robert_Collier

By writing down and collecting ideas slowly over time, working on them in small fits and spurts, when one finally comes to do the final work on their writing project or other work, the pieces only need minor shaping to take their final form. This process allows for a much greater level of serendipity, creativity, and potential sustained genius of connecting ideas across time to take shape in a final piece.

How does this relate to diffuse thinking? How can slow diffuse thinking be leveraged into this process?

Writing down fleeting notes while walking around can be valuable as one's ideas brew slowly in the mind (diffuse thinking) in combination with active combinatorial creativity, thus a form of Llullan combinatorial diffusion.

Many business books seem so shallow and often only have one real insight which is repeated multiple times, perhaps to drive the point home or perhaps just to have enough filler to seem being worth the purchase of a book.

Napoleon Hill's Think and Grow Rich is an example of this, though it shows a different form of genius in expanding the idea from a variety of perspectives so that eventually everyone will absorb the broader idea which is distilled to great effect into the title.

This quote from Twyla Tharp seems like another instantiation of Napoleon Hill's mantra "Think and Grow Rich", but is more concrete and literate: "Write and Grow Rich" (or successful, at least.)

This also sounds a bit like the general philosophy behind Napoleon Hill's Think and Grow Rich.