My understanding of ばかり so far:
After TE form, the meaning is unambiguous as it applies to the verb, e.g.:
本を読んでばかりいる。Do nothing but read books.
However, placed after the noun
The meaning is left ambiguous as it could mean
- read books only (i.e. nothing else, not magazines, not brochures etc., no implication on how much time the person spends actually reading.)
Alternatively, same as before
- ONLY read books (i.e. do nothing else BUT read, no implication on the actual material).
However, in another example from a grammar book:
It's listed as "John is drinking beer the whole time (he doesn't do anything else)."
I'm not sure whether the meaning is left ambiguous here with this particular translation picked arbitrarily (which would confirm my previous understanding) or one-way only (which would oppose it, to some degree).