Which country are you in?
They are right in the main for companies here in the UK, formal qualifications and experience is wanted. You do not need to buy extortionately priced books about ROVs.
ROvs are electrical/mechanical/hydraulic pieces of equipment just like any other machinery.
What would be best for you is along the lines of an HNC or NVQ/SVQs in electrics, electronics or hydraulics. Once you have the formal background, you then have enough of a base to learn how these things operate on ROVs. The Companys supply training for that.
Please dont spend £150 on "The Compenent ROV Pilot book" or any such thing, it really is a waste of time (and money).