I won't go into the depths of Locke studies here. Suffice to say he was the first of the 'British Empiricists'; building on Descartes' ideas and beginning an epistemology that influenced Berkely, Hume and many others. The Essay is a (very) lengthy account of his ideas - in which he begins by denying the possibility of innate ideas and goes on to explain how we come by all our ideas, discussing on the way his influential ideas on personal identity and primary and secondary qualities.
The problem that the essay has is that it's over-long (at about 800 pages) and filled with rambling repetition. Not actually amnaging to get through it all myself, I thought this abridged version might contain the highlights as it were... Well, if you have only a passing interest, this book is cheap and does set out Locke's main ideas without much repetition. For serious study, however, I'd invest a bit more for an unabridged copy (the cheapest I think is Penguin Classics; the best the one edited by Nidditch)