Microsoft Open Office XML has 6000 pages of documentation, still professionals need better documentation. Open Office XML is the largest standard that was ever put under ISO fast-track procedures.1
The large amount of comments filed by national standard bodies indicates that ECMA did no proper review of the standard proposal. No one printed them yet. However the dispositions for the comments provided by ECMA for the BRM comprise another 2300 pages of bugfixes and deprecated functionality.
While Open XML is much more feature-rich than other existing document formats, it also contains comprehensive documentation of its broad array of functions/features (both of which account for its impressive 6,000-page length), so that an experienced developer can begin to write simple Open XML applications within a few hours of beginning to read the open standard. (Microsoft Fact Sheet)
Happy hacking! But first read the spec. If its too long, don't worry. You just don't belong to the group of people who have the capacity to understand the virtues of the greatest spec of all times:
Spec writers, including the editor of the spec, the subject matter experts who contribute to the spec, … need to make sure that the spec says what it should, and doesn't say what it shouldn't. This is (or should be) a self-selecting group. If someone doesn't have the capacity to read and assimilate material, then this isn't the group for them. But such a person shouldn't complain about the length of the spec. — Eric White, Open XML evangelist
The binary legacy format to which "backwards compatibility" is sought is not part of the specification. Also the specification of an API is not made available.