How accepted is "The Decline of the West" in modern terms? As someone who enjoys history but is in no ways a professional would knowledge from this book help me in my understanding of history?
Links to previous discussions is a starting point, not to discourage more discussion, questions, data, or debate.
For discussion of grand systems of historical law, I think "Do you believe in historical laws and metanarratives?" looks good, with replies by /u/AncientHistory (who explicitly mentions Spengler as an example) and /u/depanneur . That may satisfy your immediate question of Spengler's theory, whether you should read it to learn about history.