See the sources in General: Texts & Advice. Almost all of the books and articles about scholarly writing discuss topic selection.
In addition, see:
Ruthann Robson, Law Students as Legal Scholars: An Essay/Review of Scholarly Writing for Law Students and Academic Legal Writing, 7 N.Y. City L. Rev. 195 (2004). HeinOnline | LexisNexis | Westlaw. Compares and criticizes books by Fajans & Falk and by Volokh. Emphasizes that the student writer's passion for his or her subject is of primary importance in topic selection.
LexisNexis, Researching for Law Review or Journal (2012). Short (4 page) guide discussing topic selection and preemption checking.
Heather Meeker, Stalking the Golden Topic: A Guide to Locating and Selecting Topics for Legal Research Papers, 1996 Utah L. Rev. 917. HeinOnline | LexisNexis | Westlaw.
Includes tips on how to find a topic, different types of topics (traditional and nontraditional), and how to conduct preemption research to determine if the topic you are interested in is the topic of an existing article.
Westlaw, Guide to Law Review Research (2009). A 33-page guide to selecting a topic, conducting a preemption search, developing a topic, checking citations, and related subjects.