The Essential Secrets of Songwriting" is frequently updated with tips and with songwriting analyses of hit songs ("Rolling in the Deep" by Adele is a great recent example). Some tips are quite specific and musical (chord inversions, visualizing melodies, modal chord progressions, "making a MIDI orchestra sound real"). Others are more motivation and workflow oriented (identifying goals, dealing with writer's block, starting vs. finishing songs, writing better lyrics). There is a lot of excellent and thought-provoking material on songwriting here, with a huge archive of past articles. I'm taking notes and trying some of these ideas as I am now working on some new songs for my next recording project.
So now I'm happy to have bought Gary's app, supporting a musician/writer who is providing valuable educational material to songwriters everywhere for free through his blog.