Start with learning how to properly translate 3D forms into a 2D plane. It's the most important fundamental of drawing. Drawabox was already mentioned and it's a solid resource. If you are experienced at sailing the high seas and finding stuff, the better option is Dynamic Sketching by Peter Han. Drawabox draws inspiration from it.

There's also an ongoing perspective course by Marshall Vandruff on proko.com if you can afford the price. I've looked at the free samples from it and it seems great and you will absolutely perspective for a visual novel. It's new so I don't know if you can find it from alternate sources, but if you can't and don't have the money, Dynamic Sketching or drawabox will still give you a great base.

Next up, human anatomy. Analytical Figure Drawing by Michael Hampton is a great resource. It shows you the form and placement of the major muscles you need to know as an artist one area of the body at a time. Another great teacher for anatomy is Steve Huston, try them both and see what sticks. I also like the Morpho anatomy books.

Feel free to message me if you need help tracking these down.