The main objective of The Third Manifesto, besides being theoretically sound and avoiding arbitrary restrictions and pragmatic debasement of the relational model, is to make a simple, restricted and precise definition of the role of object orientation in Database Management systems emphasising the few valid ideas from object modelling that are orthogonal to relational modelling.
External links
It has been published as a paper and as two different editions of the same book ISBN 0-201-70928-7, with http://www.thethirdmanifesto.com/ holding errata and related materials.