Not that I know the specific book, but the best advice I can give
anyone who picks up old books about Celts or anything else is simply
to read them in the context of their time and place. If you can find
Oman's name in a Who's Who or whatever and figure out a little bit
about who he was, then you can deduce what sort of agenda he had and
thus evaluate his writing. I'm not aware of any particularly
'accurate' books about the celts, especially if you go back 50 or
more years, but that shouldn't be a problem. However, one thing I've
noticed on this list is the use of texts like O'Curry's, books
written about mythology from the earlier part of this century. As
there are only a limited number of original documents in Latin and
Irish, it is best really to use the most up to date translations--the
earlier ones made up as much as they translated.
Basically, books and documents are like any other form of material
culture: they have as much to do with what's going on around them
as what's contained inside.