There is no offline manual, but there is an official documentation site at http://codex.wordpress.org/.
I travel all the time and when I need to save any web pages so I can read them offline, I simply visit the page and from the Browser's menu choose FILE > SAVE AS and type in the name of the page so I can remember what it is (some titles aren't very helpful like "home page". Whose home page?)
It's just like saving a word processing document. Put it where you can find it, and then double click on it to read it when you are offline.
There are also free and cheap software programs (check http://www.downloads.com or http://www.tucows.com) that will "grab" a whole site and download all the pages (to a point - you set how "deep" you want it to save) and then puts it in a folder to read at your leisure.
The WordPress documentation is ever evolving and improving, with new articles coming in constantly. You are dealing with open source here, and sometimes documentation is the LAST thing (other than a good readme file) that the developers get to cuz they are having so much FUN with the code. But a lot of others who don't dig into the code, with vengence and glee, are working overtime to write up as much documentation as possible.
If you are really new at this, these are the pages I recommend you check, save and read first, just to get you started.
New to WordPress: Beginners
Introduction to Blogging
First Steps with WordPress
WordPress Blog Design and Layout
Using WordPress Themes
WordPress CSS Information and Techniques
Creating individual Pages