What's a good tool for creating .epub format e-books?

Try Sigil, from here; http://code.google.com/p/sigil/

This is an excellent program for creating .epub files, works almost like a word processor

After a lot of frustration with other applications I can say this one works, has good support for .svg images and tables just code your tables as xhtml, it can easily import an existing .epub too.

Have you tried Calibre?

I'm not sure about the images, but I know that it converts formats very well, and as a bonus, works with my Nook.

I'm listing a couple of options that I've installed and used. I'm not sure of the extent of svg support, but I believe there is svg import support. Sigil is the one I'd try first. ecub is free but not open source. Jutoh is commercial (though inexpensive if I recall, and it has a trial mode for the first 20 docs), but it might be worth looking at just because of its interesting implementation (not necessarily a knock against it). It is supposed to be feature-rich, though I found it too slow on my machine at the time.

Sigil -- A WYSIWYG ebook editor.
Jutoh -- "epublishing made easy"

As was mentioned before, Calibre can also work at bridging formats. It isn't designed as an editor, just a converter, but I've used it successfully for simple documents with straightforward graphical elements.