Sites like Sparknotes are pretty widely used by high school and college students scrambling to get to class on time without reading the assigned material (or who are just trying to jog their minds a little before writing a paper), and also by teachers who didn't prepare for class.
People write those reports and submit them to the sites...your second question--do you mean what can you do to understand the book, or what can you do to understand the literary analysis?
To understand the literary analysis...well, I don't know. Read other literary analyses of the book you've just finished.
To understand the book--read analysis of it. Most classic works have had tons of commentary written about them, some by more astute or credible authors than others, but all of it can help to broaden your ways of thinking about fiction, whether fiction you're reading or fiction you're writing.