Books in English
1. The Hassidic Movement and the Gaon of Vilna by Elijah J. Schochet (Jason Aronson publishers, 1994)
Very interesting survey of the conflict and differences between the groups. Though somewhat biased toward the hassidic side, it nevertheless gives a good idea of some of the issues involved.
2. The Faith of the Mithnagdim - Rabbinic Responses to Hassidic Rapture (Johns Hopkins University Press 1997 and 1999 [hc and sc editions]) by Allan Nadler
Scholarly work on the differences between the groups, largely based on the writings of Rabbi Pinchos of Polotsk, a famous maggid, and student of the Vilna Gaon.
Books in Hebrew (Loshon Kodesh)
1. HaGaon (Mochon Moreshes Hayeshivos, Jerusalem 5762 , 3v.) by Rav Dov Eliach
Volume three contains two chapters, twenty eight and twenty nine, which are devoted to the matter. Excellent exposition from a Misnagdic point of view.
In fact, the work angered certain hassidim so much that they embarked on an extended campaign against it, including banning.
2. Chassidim uMisnagdim - Mordechai Wilensky (Mossad Bialik, Jerusalem, 5730 [first edition] and 5750 [second, revised and expanded edition][two volumes]
A study of the controversy between 1772 and 1815, when it was at peak intensity. Composed of an extensive collection of documents and polemical works from that period with introductory essays and notes.
Chasidiana - A collection of writings opposed to Hassidism
Mesora.org - A website that promulgates traditional Jewish belief
MoshiachTalk.com - A website against Lubavitch messianism