This is an excellent volume that collects Duck Variations (1972), Sexual Perversity in Chicago (1974) , Squirrels (1974), American Buffalo (1975), The Water Engine (1977) & Mr Happiness (1978). This is where Mamet's great works began, this volume focusing on his works of the Seventies, prior to the success of plays like Glengarry Glen Ross & Speed-the-Plow in the 1980's.
You can detect Edward Albee, Harold Pinter and Samuel Beckett in the early works such as Duck Variations and Squirrels (countering the opinion presented in Gary Indiana's BFI book on Salo that Mamet is just a rip-off of Sam Shepherd- not that I've worked out how that was relevant to a Pasolini film!). The first great play is Sexual Perversity in Chicago, which was abysmally adapted as the Rob Lowe/Demi Moore vehicle About Last Night (don't exchange money to watch this drivel). It is decades ahead of similar work in this masculine domain- think In the Company of Men (Neil La Bute's film even took its name from a Mamet essay) and Todd Solondz's overrated Happiness. It's very funny, quite dark and looks at the way men are...
The other major work is American Buffalo, set in a junk shop and exploring common themes within Mamet's oeuvre (masculinity, loyalty, betrayal) and is delivered in the typical rhythmic language that some people object to (as he uses words like f*** to syncopate the dialogue- jeez, this is how people speak!).
As is common with the other volumes, the smaller works you've never heard of can be just as/if not more than enjoyable than the famous plays. The Water Engine and Mr Happiness fit into this notion and sit well against such works as Lakeboat, Reunion, Prairie du Chien, The Shawl & The Woods.
This is an excellent introduction to the drama of David Mamet, which I feel is his strongest mode compared to the essays (The Uses of the Knife, Make-Believe Town), the novels (The Village), the screenplays (Hannibal, Hoffa) and the films (Things Change, State & Main).