"Modern Treaty Law and Practice" (2nd edition)

by Anthony Aust

This is widely considered as the primary English langage textbook in the field of the law of treaties.

Publisher link: http://www.cambridge.org/uk/catalogue/catalogue.asp?isbn=9780521678063

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"The concept of treaty in international law"

by Jan Klabbers

This is an excellent complement and at times antidote to Anthony Aust's text. Highly recommended.

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"Treaty Interpretation (Oxford International Law Library)"

by Richard Gardiner (Author)

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"On the Interpretation of Treaties: The Modern International Law as Expressed in the 1969 Vienna Convention on the Law of Treaties"

by Ulf Linderfalk

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"Essays on the Law of Treaties: A Collection of Essays in Honour of Bert Vierdag"

by Jan Klabbers, Rene Lefeber (Editor)

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"Contemporary issues in the law of treaties"

By Fitzmaurice, Malgosia; Elias, Olufemi

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"A Guide to the Nuclear Arms Control Treaties"

by David B. Thomson

Available in PDF format