<p>I'm planning to write an operating system and I don't know very much about operating systems. Are there any good resources or books to read in order for me to learn? What are your recommendations?</p>

Operating Systems Implementation Prentice Software
This book is written by Tanenbaum, the main guy behind Minix, which is what Linux was based on. It provides good overviews for basic OS concepts like memory management, file systems, processes, etc. The concepts in this book book are intimately tied to examples of the Minix OS, which is a good thing.
I think you should start by something like that.