Is there a good simple software to convert ebooks in txt format?

A commonly used TXT-to-pleasant-book-converter is known as a typographer.

However, since you have Linux, you may already have LaTeX installed. Though this is probably not the answer you were looking for in order to easily convert your files to pretty books, it will surely fix the problem with a little work and learning.

LaTeX WikiBooks is a nice place to learn what it’s all about.

Try calibre. It’s already in the software repositories of several major Linux distributions. According to the Ubuntu package, “[i]t includes library management, format conversion, news feeds to ebook conversion as well as e-book reader sync features” and supports “MOBI, LIT, PRC, EPUB, ODT, HTML, CBR, CBZ, RTF, TXT, PDF and LRS” formats.

If by “simple” you don’t mean “GUI” but “command-line”, you might instead try one of the following programs. (Install, if necessary, the corresponding package and read the manual.)

fmt: “simple optimal text formatter”
This is the classic Unix/Linux plain-text reformatter. It actually lives in the coreutils package and is therefore almost certainly already installed on your system.

par: “filter for reformatting paragraphs”
Like fmt on steroids, it produces prettier output. It has lots of options, and the manual requires multiple readings, but it‘s also very clever and might do what you want by default.

reformat: “tool to simple format plain ascii texts”
At first glance, this appears to have fewer features than fmt, but you might give it a spin.

highlight: “a universal sourcecode to formatted text converter”
This is designed for source code, but it has a setting for plain text input.

Thanks for your answer! I use eeebuntu, btw.