Peg,
Yes, every history book is its author's interpretation of the data. No, not every book is equally close to the truth. If one book states that the nuns of Kildare truly, to the best of their ability, worshipped the one God to the exclusion of all others, and another book says that they only pretended to do so while secretly and knowingly departing from Christian practice by worshipping other beings that they regarded as gods equal to or superior to the God of the Christians, then both books can't be right according to the same terms of reference.
T.