Welcome!
And what RWood said... Both libprs500 and Book Designer are excellent tools. If I were just looking for a straight .txt file converter, I use Book Designer. It has a learning curve (especially if you want to use advanced functions - I'm still learning what it can do.) But both are excellent.
You may want to reference
this entry on the Wiki about Book Designer, or look at Kovid Goyal's
website for details on libprs500. (We really should get a Wiki page started for that...
) And of course, the relevant threads in the Reader sections here - though they are getting a little long in the tooth size wise.