Ask HN: Resources for Decoupling Transactional Systems

I'm trying to untangle a hodge podge of transactional systems that are married, data-wise, together and bolted onto a 25 year old ERP system in the hopes that we can piece-meal a migration to new platforms.

One of the challenges is that a lot of the analytical reports and data come from the transactional systems of record.

I'm aware of the need to have a Data Warehouse / OLAP that feeds off the transactional systems.

Another problem is that we have disparate systems like a CRM and a mobile application that share data with the ERP (customers, etc) and it's hard to decouple.

I guess I'm really looking for some literature if anyone can recommend that deals with solutions to this problem: untangling transactional systems and migrating them to new platforms. Not a cookbook but maybe best practices.

We've started evaluating the data dependency from the database and web service perspective of these married systems as a first step.