While I'm not sure there is any such thing as "best book" seeing as I have many, many favourites, I would highly recommend that anyone who has reached an advanced level in their English studies to read "Grammar Snobs Are Great Big Meanies: A Guide to Language for Fun and Spite" by June Casagrande. It does a very good job of laying out how many of the grammar "rules" that trip up even native English speakers are arbitrary and which "rules" you can safely ignore. Generally if it feels and sounds right then you are using the right phrase. It's very funny and highly engaging and I know that a lot of friends who are native speakers but who have always been uneasy about their grammar found it very useful. That said I would be remiss if I didn't say that Terry Pratchett's Discworld series isn't the greatest book series I have ever read. (But I would not start at the beginning. It is divided into 6 sub-series based on topic and I advocate starting where your interest lies. My recommendations would be the Death series (Death is hilarious), the Watch, or the Industrial Revolution.) I found it because I watched the made for TV movie "The Hogfather" based on one of the books from the Death series, Hogfather, and was hooked. A guide to the subseries: https://io9.gizmodo.com/how-to-read-terry-pratchetts-discworld-series-in-one-h-1567312812