Writing the annotation is the most difficult part of creating an annotated bibliography. Creating a bibliography in itself is fairly straightforward and is described in numerous writing style manuals, including the APA's and the MLA's style manuals. A writer needs only to be careful in documenting the facts about the source being listed and in properly formatting the information about the source to create a simple bibliography or reference list.
Providing annotations for the sources is a different thing entirely. The researcher must not only identify the sources, but must also read them in their entirety or at least review key portions of the entire works to arrive at a critical assessment of the value. Although the annotation might include information on article or book content, one of its aims is to evaluate the article or book. Things to observe might include the availability of statistical data, inclusion of photographs or illustrations, author's qualifications, presence of a reference list, historical significance of the material, etc. In short, review the source's content and features and evaluate it in relation to the topic you are covering. A collection of essays may only have one relevant essay. In evaluating the collection, you need only comment on the relevant information rather than try to cover the entire book. If an entire book has information relevant to your topic, hit the highlights rather than trying to cover the entire book in an annotation. Comment on the most useful chapters, on features of the book that proved useful (references, illustrations, statistics, etc.), and on the author's qualifications for writing such a book. For journal articles, focus on author qualifications, special features, recency, and references.
Keep in mind that the overall purpose of the annotated bibliography is to provide other researchers with enough information about the sources you've gathered to help them evaluate and select from among them.