Calibre is a program in active development (note the 0.x numbering). There is generally a new public build released every week. Some of them add major features and some fix minor bugs; check the changelog to see what any given update does. Every so often, a significant bug does get in; if so, there will be bugfix releases outside the normal schedule.
I've been using calibre a bit over 4 months, since 0.6.41. The number of features that have been added in that time is just amazing. So, grab the latest version, install same, and keep on keepin' on. You'll get used to doing this on a weekly basis.