It seems like there isn't. Microsoft don't have plans to write one and it's a Microsoft proprietry closed standard with DRM embedded, so I don't think anyone could develop a reader easily. I don't think anyone but Microsoft have developed a reader for this format even on Windows.
If you've bought digital books from vendors and downloaded the .lit version, you may be able to ask for a .pdf (Adobe Acrobat eBook) format in exchange. There's no problem with this format on a Mac.