@NiharikaSarma already covered some of the best books about writing. I enthusiastically second Elements of Style for practical grammar and good style guidelines to follow. Another good one is Bird by Bird by Anne Lamott.
The best way to improve is to write a lot and read a lot. There's not really a way to get around that.
For dialogue, besides reading good fiction, I would recommend listening to people's conversations and actively observing your own conversations. Most people don't speak in complete sentences and use a lot of fillers such as "um, like, mm" etc. I don't know of any books about dialogue, but as Stephen King says in his On Writing memoir, most books about writing are BS, the best way to learn is to do it. Not that other books recommended can't be helpful but they are only tools and nothing more.
Side note: I prefer the terms "gardener" and "architect" to pantser/planner because they are a bit more descriptive, and I don't really like the negative connotation of "pantser." Also GRRM, who coined "gardener" and "architect", is a gardener. One is not better than the other and you will probably need both to write a good story.
Writing is a strange magic that consists of hard work and putting one word in front of the other. It is very difficult but very worth it, and the process is different for everyone. Best of luck to you friend!