Bluefire can be used to download library books directly, via a mobile Safari 'bookmarklet' workaround (that changes the default behavior of the Overdrive web portal). I much prefer Bluefire to OMC (OMC has no night mode, landscape reading, 'chrome' does not auto-hide). See Liz Castro's blog,
Pigs, Gourds, and Wikis.
Basically once you land on the download page, you run the bookmarklet, and now the Download button brings up a dialog that allows you to select Bluefire as the target app.
The blog describes the procedure for iPad, but it will work with iPad/iPhone as well - provided you use the 'full', non-mobile Overdrive library portal. Seems like it should be possible to do a similar workaround for the mobile portal as well, and I'm hoping to figure out what to modify in the bookmarklet Javascript code to make that work.
Note that OMC doesn't seem to support 'early return' of library loans, even where the licensing should allow this. Bluefire does. Another reason to use BF. Of course OMC is needed for audiobook loans (which don't allow early return on the basis of the license agreements). Hopefully Overdrive will fix this and improve the reader over time, but I won't be waiting for that to happen now...