Translation:The girls like books about animals.
Why is it "leis" rather than "leo"? I would have expected 'the girls' to use third person plural.
leo = with them
You don't want to say "with them the girls"; you just want to say "with the girls." Usually, "with" is "le," but in front of "an" and "na," it becomes "leis," so it's "leis na cailíní."
Unfortunately for us learners, there is another "leis," the prepositional pronoun which means "with him," and that's probably what you're thinking of. You can read all about "le" here: http://www.nualeargais.ie/gnag/gram.htm
The 'prepositional pronouns' (fúm, fút, fúthu, etc.) are used when the object of the preposition is a pronoun (faoi + mé = fúm, faoi + siad = fúthu, etc.). Before other nouns the ordinary preposition is used, so faoi ainmithe.
In the case of some prepositions, the 3rd personal singular masculine form of the prepositional pronoun (+him/sé) is the same as the simple preposition: as, de, faoi.
Leis is the form of le used before the article, leis an/leis na, AND the prepositional pronoun meaning 'with him' (le + sé).
So can some help with link for the difference in pronunciation between leabhar and leabhair. Based on what i've learned thus far is that thw only difference is the the r making the word mean book is round, while the r make the word mean books in a tap/flap. Is that it? Or is there a suttle change in the vowel sou d as well?