"The book is about a horse."
Translation:Το βιβλίο είναι σχετικά με ένα άλογο.
Yes, it is. It's like saying 'this is in relation to that', you can't just say 'this is in relation that''. Σχέση = relationship, so maybe that's how you can remember it. :)
AS you can see from the heading on this page the preferred translation is ... "σχετικά με..." Για is also a possible translation.
I wrote μ´ένα instead of με ένα, but it said is wrong... is bc of the tone on the ε?
It's almost correct, it had not been included in the possible translations (it's kind of impossible to include such alternatives for every sentence, but it will be added for this one now that you mentioned it). Just use ( ' ) instead.