Books can offer an in-depth, even comprehensive view of a subject. Articles, on the other hand, will be more focused on a specific aspect of a subject.
For example, suppose you are writing a paper on a poem by Robert Frost. You can easily find a book on the poetry of Robert Frost in the library's collection, but that book might not discuss your particular poem in detail. However, you could probably find several articles that focus specifically on the poem. Articles are great sources for finding information on individual facets of a topic.
There are few websites that can boast of being comprehensive. For the most complete coverage of a subject, you will usually want to consult books, then periodical articles.