Eugene Volokh, Academic Legal Writing: Law Review Articles, Student Notes, Seminar Papers, and Getting on Law Review (5th ed. 2016), RESERVE KF250 .V65 2016. Part I of the Volokh text discusses "Finding What to Write About (The Claim)."
Elizabeth Fajans & Mary R. Falk, Scholarly Writing for Law Students: Seminar Papers, Law Review Notes and Law Review Competition Papers (5th ed. 2017), RESERVE KF250 .F35 2017. In particular, see Chapter 2, "Inspiration: Choosing a Subject & Developing a Thesis"
Heather Meeker, Stalking the Golden Topic: A Guide to Locating and Selecting Topics for Legal Research Papers, 1996 Utah L. Rev. 917. Available from HeinOnline, LexisNexis & Westlaw.
Richard Delgado, How to Write a Law Review Article, 20 U .San Francisco. L. Rev. 445 (1986). Available from HeinOnline.