Edward K.
Watson, author of Mormonism:
The Faith of the Twenty-first Century (1998) has an abbreviated version
of his book critiquing the Iglesia Ni Cristo:
I have not
encountered many INC members (except some on facebook), but they have a large
presence in the Philippines (where they were founded by Felix Manalo), though
they seem to have some presence in nearby in two cities in the Republic of
Ireland (Galway and Dublin) and also in Belfast in Northern Ireland. It has a good discussion of proof-texts the INC uses to "prove" the Bible prophecies of Manalo, including Mal 4:5-6, and a section devoted to the Holy Spirit being a person.
Notwithstanding,
their Christology is very similar to that of the Christadelphians (i.e.,
Socinian), a group I have written quite a deal with (see my Listing
of Articles on Christadelphian Issues which discusses John 17:5 and other texts pertinent to the nature of Christ's pre-existence).