hiya! I dont know about spark notes plus, but litcharts is a good website for english, but some stuff is locked behind a subcription which i think was about 9 pound a month. But once you get the subcription you can download the pdfs and they'll stay in your folders so really it can be 1 time purchase thing
the app Knowunity has some notes that people upload, it could be worth checking out (its free too) i've got some kite runner notes saved there that are quite good, its got groupchats too to discuss subjects that can be quite good if you've got questions or if you want someone to read over your work
hope this helps!