The MHRA Style Guide Online
A Handbook for Authors and Editors • Third Edition
From Chapter 4, 'Abbreviations'
4.5 American States
The two-letter postal abbreviations for American states, e.g.:
CA (California), IL (Illinois), MA (Massachusetts), NY (New York)
have largely replaced the official abbreviations, though a few of these, in particular ‘Calif.’ and ‘Mass.’, are still widely used. The postal abbreviations, which have no full stop, should be used whenever it is necessary to include the name of the state in bibliographical references (11.2.2, list of items, no. 7). These abbreviations are given in the New Oxford Dictionary for Writers and Editors.