A few that I've enjoyed-

The Humours of Planxty by Leagues O'toole, an admiring look at the history of the band with interviews of it's members.

In Search of the Craic: One Man's Pub Crawl Through Ireland by Colin Irwin. Fun and informative.

And the most invaluable book I've found for instruction, Celtic Back-Up for all Instruments by Chris Smith. It helped me with theory and got me started with playing counterpoint to Irish and other kinds of traditional music. Lots of good Irish tunes along with interesting but appropriate chords, as well.