Please suggest good book on Celtic monsters

Can any Doper please recommend a good book or books on Celtic mythology, especially ones that give a good bestiary of Celtic monsters and/or include information about the Ifrinn?

I am customizing my D&D campaign, which is set in an alternate British Islands, and I need more Celtic monsters, especially those that may be found in the Ifrinn (a hell of Celtic myth).

I am asking for book recommendations because I have run several web searches and have found some information, but not nearly enough.

Thank you very much.