Been reading Lifeblood (Darkside 2) with kiddo for the past few days. Also beta-reading for a fellow writer, and picked up Neil Gaiman's Norse Mythology. It's slow-going between the three (and other stuff) but luckily they're all good reads.
The Irish mythological cycle and Celtic mythology
by Arbois de Jubainville, H. d' (Henry), 1827-1910; Best, R. I. (Richard Irvine); United Irishman
Book is old so it is in public domain. I found it by searching for Fomorian invasions.