Board Thread:Questions and Answers/@comment-26858522-20151206180900/@comment-6697950-20151206182219

The anime doesn't invent an original story or rewrite it, if that's what you had in mind. There are differences between the novel and anime, most of which are content cuts or some minor original scenes, and all these notable differences are listed on the respective episode pages.

If you've watched the anime to the end, you should be able to skip the novels that were adapted into anime format if you want, but you will miss out on some not-so-important-but-interesting world building, monologues, characterisation and similar things. The majority of these losses don't make later volumes incomprehensible, so it's just a matter of how interested you are in the details.

Alicization begins with volume 9. Though I do have to warn you that you won't make much of any sense of the prologue at first, even if you have read the previous volumes, since it's only explained later on in the arc. Just continue reading and all will make sense. Or if you're too impatient, the Alicization Arc article has an introduction of the arc that explains what's going on for readers that have not reached the later volumes yet. Read at your own risk, since the introduction spoils some of the early plot points of the arc.