"σχετικά με" translates to: concerning, in regard to, deals with, etc it's all part of the phrase.
Ι would prefer the sentence "το βιβλίο αναφέρεται σε ένα άλογο"=the book refers to a horse, or "το βιβλίο αφορά ένα άλογο"=the book concerns a horse. This sentence is a bit anglicized, but it is correct though.
Thank you, that sounds like what we should use. We'll put it together witht he report for when the revision is done.
But please leave "the book is about a horse" as an alternative ... that is absolutely what We would say in (British) English ... "refers to" or "concerns" sounds a bit formal or stilted (to my ears).