Hmm, for a very simple wiki-like notebook app on Linux, Zim might be my speed
http://www.zim-wiki.org/manual/Start.html
It's super basic and it stores text in Markdown in .txt files. Pages are text files with that name. 'Sub-pages' are text files inside folders of that name
That seems sufficiently robust for me, and still a mild step up from my current format, .txt files in Gedit.