I can't use the second converter, because Polish language doesn't use Cyrillic alphabet; but I'll try the recode way soon. Thank you for the information
EDIT: Unfortunately, the Windows version of recode doesn't seem to be available for download anywhere - the download link just points to download page of GnuWin projects, and recode is not on there. Googling for it I encountered only some threads asking for it as well. I'll have to look for some other tool to do the job.
EDIT2: I managed to download the recode tool for Windows from here:
http://eloquence.marxmeier.com/sdb/html/899906321.html
I haven't figured it out yet, though... it doesn't seem to support Windows codepage 1250, which is most common encoding for Polish books ( see here:
http://www.borut.com/library/s_1250.htm )