Uairibh, you have us all puzzled. I wonder how reliable your memory is about what yer man actually said.

1- I’m reading (the) books – Tá mé ag léamh (na) leabhar or Tá (na) leabhair á léamh agam.

2- The use of ag instead of a where the verbal noun is preceded by its direct object seems to be a common mistake at all levels from beginners to native speakers:

Cad é atá tú ag déanamh? instead of Cad é atá tú a dhéanamh? (or …á dhéanamh agat).

– the books that I’m buying/reading – na leabhair atá mé ag ceannach/léamh instead of na leabhair atá mé a cheannach/léamh (or of course …atá á gceannach/léamh agam).