The Indigo BookSprigman et al., The Indigo Book: A Manual of Legal Citation, Public Resource (2016).
The BluebookThe Bluebook is a legal guide for legal writing. The "blue pages" are guides to citing legal documents in a legal setting. The "white pages" are guides to citing legal documents in an academic setting. The Bluebook offers a free trial period.
A Short & Happy Guide to Legal Writing by Deborah BormanBorman offers a short history of the practice of legal writing before diving into legal construction and framework. With examples of legal writing included in this short book, this is a great starting point for understanding legal writing.