If you have interests in songwriting I highly recommend songbook collections for the Beatles, Hall & Oates, Billy Joel, Elton John, David Bowie, and Nat King Cole or Christmas ballads for dipping a toe into postwar jazz harmony. Formal textbooks are just not going to have perfect and complete songs with adventurous chord progressions and memorable melodies.

The Beatles don’t have my favorite songs but arguably every vital lesson is somewhere in their 1964 to 1967 output. If these aren’t cool, hide them under your bed or discard as completed homework.

After a few months of digesting songs, build a habit of figuring out the key of each section, writing the chords using Roman numeral analysis, and maybe writing the melodies using scale degrees. Like, Happy Birthday ends with:

       I            IV           I        V  I
Happy birthday dear David. Happy birthday to you.
5  5  ↑5   3   1    7 6    ↑4 4  3    1   2  1

The goal is to think relatively to allow combining techniques (stealing) from any key and to be able to learn something from every song you hear.

I   vi    iii         V7/IV IV        I6
Somewhere over the rainbow  skies are blue.
1   ↑1    7 5  6   7   1    ↓1    ↑6  5

I      bVII V/ii                 ii
3  ↓5       6  #1 3   ↑6  5   3  4   6
Crazy.      Crazy for feeling so lonely.