I'll agree on Minix. Also the book:
is actually fairly comprehensible. (Unlike some books on similar topics, thinking of "Understanding the Linux kernel" here.) It's quite expensive, but you can probably pick up an earlier edition second-hand.
(NB. I'm not in any way affiliated with the book, it's just good :) )
I had previously bought these books on recommendation for the same purpose but I never got to studying them myself so only take them as second-hand advice.
Operating Systems Implementation Prentice Software
alt text http://ecx.images-amazon.com/images/I/3125C47MR4L._SL500_AA180_.jpg
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.
alt text http://ecx.images-amazon.com/images/I/3125C47MR4L._SL500_AA180_.jpghttp://www.amazon.com/Operating-Systems-Design-Implementation-3rd/dp/0131429388
Pretty solid stuff.