Joe Schall's book, Writing Recommendations: A Faculty Handbook, is available online through Penn State's e-Education Institute. The handbook provides sample letters of recommendation as well as advice about ethical and legal issues involved in writing letters, tailoring letters for specific disciplines or programs (such as law school or med school), and so on.
There are two versions, a printable/downloadable version and a version that can be accessed and searched directly online. In addition to a link to each chapter, each chapter is further divided into sections with respective links. There is also an internal search engine, which you can use to find specific terms.
Especially useful is Chapter 6: “Writing Recommendation Letters for Students Seeking National Scholarships.” For example, the chapter includes specific advice about how to write a recommendation for the following graduate fellowships: Fulbright, Gates-Cambridge, Marshall, Mitchell, and the Rhodes.
Please also see Manya Whitaker,"Tips for Writing Recommendation Letters," Inside Higher Ed December 12, 2016.