I've compiled a list of resources that I thought would be pretty helpful, starting with some books:
Draw your own manga- Haruno Nagatomo
How to Draw Manga Vol. 30 Pen & Tone Techniques
Manga in Theory and Practice- Hirohiko Araki
Making Comics- Scott McCloud
Perspective for Comic Book Artists- David Chelsea
My Road to Mangaka- A fun autobiographical comic by Mizukami Satoshi
There's also The Complete Guide to Self Publishing Comics and The Webcomics Handbook featuring examples from some not so well respected authors, but solid advice overall.
The first two books on the list are pretty dated (30 or so years old) but offer solid advice for manga, especially if you are taking pen and paper as opposed to pen and tablet approach. A few things not accounted for though:
Pages typically have a "safe zone" then a margin that extends to an area called the "bleed zone" Pages are cropped to this margin when they are printed.
If you anticipate publishing work, you have to account for the binded area. The Nodo zone is the side where the pages are bound, you don't want your text to run outside of the Bleed area here or it will get lost within the book's binding.
72 is typical for web, but if you expect to print, make sure you keep resolution to at least 300 dpi, or 600 dpi. Do it even if you don't know if you'll print so you're not kicking your ass later.
Manga in Theory and Practice is an extremely unique insight into the overall process of writing and drawing manga, but applications can go outside the the manga sphere. That in mind it was written for the audience of aspiring Shonen authors so some advice can be taken lightly.
-thousands of free fonts, all made by one guy. Purchasing licenses for the fonts are more of a kind suggestion, but consider it for his hard work.
Episodes of Manben, a program by author Naoki Urasawa that shows you how the sausage is made. He interviews different authors giving you a peek at their process.
a website that lets you compare the heights of different people. there are also some objects you can add to scale.
feel free to add anything to this list you find beneficial.