While abbreviations are rare in academic prose, they are perfectly common in citations.
Abbreviations that consist mostly of capital letters don’t need periods or spaces:
However, for people’s initials you can add periods and spaces unless the entire name is abbreviated:
C. S. Lewis
R. A. Shoaf
You’ll find though that the spaces between initials are often omitted, as anyone researching G.I. Joe would soon discover.
Lowercase abbreviations usually end in periods:
Lowercase abbreviations of multiple words may contain extra periods:
As has been pointed out elsewhere, however, this rule does not apply universally. An exception would be mph (miles per hour), which does not need extra periods.
Any months longer than four letters can be abbreviated:
Here’s a list of other abbreviations used in MLA citations:
ch. = chapter
dept. = department
ed. = edition
e.g. = for example (from the Latin exempli gratia)
et al. = and others (from the Latin et alii)
etc. = and so forth (from the Latin et cetera)
i.e. = that is (from the Latin id est)
no. = number
P = Press
p. = page
pp. = pages
par. = paragraph
qtd. in = quoted in
rev. = revised
sec. = section
trans. = translation
U = University
UP = University Press
vol. = volume
For more information on abbreviations, see section 1.6 of the MLA Handbook (8th ed.).