I bought a book called "How to be a Successful Songwrite" in Toronto in 1986. It was a series of interviews with Neil Sedaka, the Sherman Bros, and names that were unfamiliar but their songs were legion. It is out of print now, but get an archive copy through your library service - it will pay dividends. The two recurrent themes amongst writers were 1) While on a long boring car journey (mindspace) 2) what I called TWIST - Tested Words In Substitute Tune - write new tune to familiar song (or new words to established tune) and when knocked into shape change the old words/tune into a new one.
there were other gems like watching Soap Operas ie full of trite lusting, sack hopping and unrequited longing - cf 95% of popular song.
Another book to go for (published 1977 ish) is "Creative Dreaming" by Dr Paticia Garfield PhD - all about marshalling the sleeping hours for creativity, basically immerse yourself in the facts, concentrate on them as you fall asleep and have a notepad and pencil (or pocket memo) by your bedside at all times (and in the first hour, at least, of the day in my experience).