Notilo is a personal information manager; a plain-text editor with hierarchical structure. It supports Unicode. It is written in Java, so you can use it on many platforms. (I develop it on MS Windows, so this is where it works out of the box; on other platforms, some adjustments are probably necessary.)
You can download newest version from SourceForge.net.
Later I will write here some help information about using program. But it is really simply -- just create a structure of pages, and then edit text on each page. Page is saved automatically when you move to other page, or when you close book or exit program.