Likely the preeminent source for definitions of words in the contemporary American English language. Orginally based on the landmark Webster's Third New International Dictionary of the English Language, but in a constant state of revision and updating, with fascinating features on new meanings and popular searches.
The link above connects only to the printed 3rd edition, shelved in Leyburn Library's Reference collection @ PE 1408.B883824 2013. W&L no longer has online access to the third edition (2013) of this respected guide to writing for college students, but an individual purchaser of the guide can set up his/her own online access.
[Link goes to library catalog.] Seventh edition of this valuable guide is available only in printed form only -- no online access.
Copies in Leyburn Reference and Science Reference @ PE 1408 .H2778 2011
Online access to the 1918 edition of William Strunk's legendary guide to the "principal requirements of plain English style," later revised by his former student E. B. White and available at W&L in multiple printed editions.
2015 "anti-handbook handbook" presents and analyzes three hundred sentences drawn from the printed works of a single, typical day in the life of the language -- December 29, 2008. You may preview the contents.
2014 guide for serious students ranges in coverage from "sentence structure and grammar, to big-picture issues, such as adhering to genre patterns for successful research and publishing and developing productive and rewarding writing habits."