While doing it's background downloading thing, Calibri launches some process called calibre-parallel. After this process launched on my system, and begun doing it's thing, it immediately chewed up 53,210 Win32 Handles!
I was able to quit that particular process, while the main part of calibre remained running, and my Win32 Handles went back down to normal.
I'm not sure exactly what that process is used for, downloading or fetching news I think. My system is a XPS M1730 2.4GHz Intel Core2 Duo with 2GB RAM running WinXP Pro SP3.
I also filed this as a ticket on the web site, only because I couldn't decide which way to do it. I thought that here anyone else could experiencing something similar could pipe up with suggestions, whereas I saw it more of a bug so I also thought it should go in the ticket system. Since I couldn't decide for sure, you're getting both.