I just finished breezing through Kendall Gelner’s 87-page Aperture Adjustment Tool Guide, which is a downloadable PDF that you can grab for free, and if you find it useful, then make a donation. Kendall has put a lot of work into this guide, and I found it to be quite comprehensive.
One of the things I especially liked about this PDF was the many illustrations with callouts. Photographers tend to be rather visual creatures, and Kendall’s approach seems right in step with that. It’s a helpful document, and certainly worth a look.