Free Online Resources
The Stanford Encyclopedia of Philosophy contains a solid article on A.J Ayer and his philosophy. The article discusses most of the important work undertaken by Ayer, and includes a useful section on meaning and truth as discussed by Ayer in his 1936 book Language, Truth and Logic.
Squashed Philosophers: Ayer, written by Glyn Hughes this page aims to take the all important points of Language, Truth and Logic and present them in a way which will allow the reader to get to grips with the book within an hour. Glyn Hughes should be commended for what is a rather good abridged version of the book, though of course I still recommend that one reads the full book.
There are two separate Wikipedia articles that may be of interest to those who are studying Ayer. Both are rather sparse and contain relatively little philosophical content, but they still may be worth a read. At the time of witting it appears that both articles are also free from any major blunders and misrepresentations. Language, Truth and Logic and A.J Ayer on Wikipedia.
A series of lecture notes on Ayer's Language, Truth and Logic can be found online in PDF form. I believe that these lectures were written by Oxford Philosophy faculty member Peter Kail and they provide a great introduction to some of the ideas featured in Ayer's work. The lecture notes can be found at the following locations Lecture I: Introduction, Lecture II: The Verification Principle, Lecture III: Definitions, Reduction and The Verification Principle, Lecture IV: Analyticity, Confirmation and Holism, Lecture V: Critique of Ethics and Theology, Lecture VI: Minds.
An article called A.J Ayer's Philosophy and It's Greatness can be found on UCL's website. Written by Ted Honderich the article provides a broad survey of Ayer's work as well as providing information on the general philosophical background in which he is work set against.
Books on A.J Ayer
A.J Ayer by John Foster is part of The Arguments of the Philosopher series which outlines the arguments of various influential philosophers and critically discusses the said arguments to an academic level. The first half the book is dedicated to Langauge, Truth and Logic, while the second half of the book discusses Ayer's later work. This book is currently the most conclusive book of the Philosophy of A.J Ayer and is unfortunately unavailable in paperback form at the time of writing.
Another notable book worth mentioning is Ben Rogers brilliant biography of A.J Ayer which I would recommend to anyone who is interested in the life of the great Philosopher. The book can be picked up second hand for a couple of pounds on Amazon.
The Philosophy of A.J Ayer edited by Lewis Eden Harris appears to be a very comprehensive survey of Ayers work featuring a total of 24 essays. Ayer himself replied 20 of the 24 essays contained in the book. As I'm not familiar with the book and due to the fact that it is currently out of print, I'm going to hold short of recommending the book.
Academic Essays/Papers on A.J Ayer
All of the following essays can be found on JSTOR or for free elsewhere online, while the resource isn't free many Universities have access to the service.
Anthony Quinn provides a great biography of A.J Ayer which can be found online for free and was originally published in the Proceedings of the British Academy 94, 1997. This is perhaps the best freely available biography of A.J Ayer.
Ayer's First Empiricist Criterion of Meaning: Why Does It Fail? is a four page journal article written by Philosopher David Lewis. In the article David Lewis outlines why the first formulation of the verification principle by Ayer fails due to the fact that it lets in patent nonsense. Available on JSTOR.
Metaphysics and Meaning is a 1935 paper written by W.T Stace. Here Stace attacks Ayer's 1934 paper entitled 'A Demonstration of the Impossibility of Metaphysics'. Stace makes a number of thoughtful objections throughout the 22 page paper. It should be noted that Ayer responded to many of Stace's objections in another paper the following year. All of which can be found on JSTOR.
Some Consequences of Professor A.J Ayer's Verification Principle is a 1950 paper written by D.J Connor which goes through some of the implications of Ayer's verification principle.