Update link for Intro to IR, and add for MIR & MG.

concepts/introduction.rst

@@ -102,6 +102,6 @@ References
 
 * "Information Retrieval" by C. J. van Rijsbergen is well worth reading. It's out of print, but is available for free from the `author's website <http://www.dcs.gla.ac.uk/Keith/Preface.html>`_ (in HTML or PDF).
 * "Readings in Information Retrieval" (published by Morgan Kaufmann, edited by Karen Sparck Jones and Peter Willett) is a collection of published papers covering many aspects of the subject.
-* "Managing Gigabytes" (also published by Morgan Kaufmann, written by Ian H. Witten, Alistair Moffat and Timothy C. Bell) describes information retrieval and compression techniques.
-* "Modern Information Retrieval" (published by Addison Wesley, written by Ricardo Baeza-Yates and Berthier Ribeiro-Neto) gives a good overview of the field. It was published more recently than the books above, and so covers some more recent developments.
-* Introduction to Information Retrieval (published by Cambridge University Press, written by Christopher D. Manning, Prabhakar Raghavan and Hinrich)
+* "`Managing Gigabytes <http://people.eng.unimelb.edu.au/ammoffat/mg/>`_" (also published by Morgan Kaufmann, written by Ian H. Witten, Alistair Moffat and Timothy C. Bell) describes information retrieval and compression techniques.
+* "`Introduction to Information Retrieval <http://www-nlp.stanford.edu/IR-book/>`_" (published by Cambridge University Press, written by Christopher D. Manning, Prabhakar Raghavan and Hinrich) is available both in print and for free online.
+* "`Modern Information Retrieval <http://www.mir2ed.org/>`_" (published by Addison Wesley, written by Ricardo Baeza-Yates and Berthier Ribeiro-Neto) gives a good overview of the field. It was published more recently than the books above, and so covers some more recent developments.