It may be argued that narrow reading produces only the ability to read in one area. This is not true. Deep reading in any topic will provide exposure to a tremendous amount of syntax and vocabulary that is used in other topics. Any technical field, for example, will use "subtechnical" vocabulary, words such as "function," "inference," "isolate," "relation," etc. (Cowan, 1974). Also, readers typically do not read only one author or in one area for the rest of their lives; they gradually expand their reading (for evidence that high school students gradually expand their reading interests as they read more, see LaBrant, 1958).
The clearest advantage of narrow reading, however, is that it is potentially very motivating. In any anthology, it is certain that most topics are not of great interest to most readers. The combination of new vocabulary, unfamiliar style, lack of context, and lack of interest in the subject matter insures that much reading remains an exercise in deliberate decoding. In contrast, narrow reading on a topic of real interest has a chance of resulting in the reader really reading for the message, for meaning, in early stages of language acquisition.
There is some evidence supporting the narrow reading idea. Lamme (1976) found that good readers in English as a first language tended to read more books by a single author and books from a series, a result that many readers of this paper can identify with, former devotees of Nancy Drew, the Hardy Boys and Bobsey Twins. More recently, Cho and Krashen (1994, 1995) reported considerable enthusiasm for reading and substantial vocabulary development among adult second language acquirers who read books in the Sweet Valley series; readers rapidly moved from Sweet Valley Kids (second grade level) to Sweet Valley Twins (fourth grade level) to Sweet Valley High (fifth and sixth grade level). Several readers in this study had never read a book in English for pleasure before, but became fanatic Sweet Valley fans.
Here are some suggestions for those who want to see for themselves if this works, who want to try narrow reading in a language they have some competence in, but want to improve in:
Lower your standards. Read only material in the second language that is genuinely fun and interesting, material that is so easy that you probably feel guilty reading it in your primary language. This is your excuse to read comics, magazines, detective stories, romances, etc. There is no shame in reading translations.
Reading at this stage does not have to make you a better person, does not have to give you insight into other cultures, and does not have to improve your knowledge of history or science. But if you do enough narrow reading, you will be much better prepared to read "demanding" texts.
Don't worry about pushing ahead rapidly to harder and to different material. This will happen on its own. The best way to expand might be a gradual movement from one field to a closely related field, taking advantage of the overlap in context and language.