I liked parts of this but I'd be hesitant to recommend it to beginners. First and foremost, the mythology was completely all over the place. "Celtic" gods is an inaccurate descriptor because "Celtic" refers to a wide range of cultures, and this book was conflating Irish, Gaulish, and other deities. It also referred to Irish deities of aspects of each other - such as Donn being an aspect of An Dagda (who in turn was an aspect of... Jupiter?). Donn and An Dagda aren't even members of the same race. From Lebor Gabála Érenn, An Dagda is a member of the Tuatha Dé Danann, and Donn is a member of the Milesians who conquered Ireland after the Tuatha Dé Danann. In fact, he was fated never to step foot on Ireland. It also uses conventions such as "Celtic god of..." and "Irish god of..." when these are not applicable to Irish deities, who have many complex, multi-faceted aspects (I'm thinking specifically of the repeated reference to Aonghus as the "Celtic god of love." Aonghus is Gaelic.)
The information on folk practices was fascinating, but it bounced back and forth a lot where in one sentence you'd be learning about Irish, then the next Welsh, then back to Irish, then Scottish, so it made me a little dizzy. On top of that, it references Robert Graves who is... a nutter and Peter Beresford Ellis whose scholarship tends to be questionable.
Definitely going to save this for my own reference, but if you're looking for mythology, you should start by reading the source material yourself for sure.