I'm going to recommend Michael Dobbs' Down with Big Brother: The Fall of the Soviet Empire to you. It's extremely readable, relatively cheap (Amazon has it for $12 for the Kindle edition) and well-researched.

In short, Gorbachev thought Vladimir Aleksandrovich Kryuchkov (appointed KGB head in late 1988) was utterly reliable to the state, regardless of perestroika. "He had every confidence in Kryuchkov's loyalty, seeing him as a consummate acolyte, who had spent his entire life fulfilling the wishes of his superiors. It was difficult to imagine such a creature leading a coup against his commander in chief. When he was eventually presented with evidence of Kryuchkov's betrayal, he initially refused to believe it."

Dobbs makes the case that Kryuchkov at first attempted to maneuver Gorbachev into taking a hard line. Such hardline approaches had worked in Hungary, Czechoslovakia and Poland (in 1981), and he believed that was the case in 1990 and 1991.

Instead, Kryuchkov and the KGB badly bungled their intervention in Lithuania. Gorbachev had said publicly that he would not use force in that country, but after the KGB intervened -- likely exceeding orders -- he stood by Kryuchkov and the KGB and refused to apologize. This caused a severe break between Gorbachev and the liberals. In an effort to compromise, Gorbachev tried to walk a fine line and failed -- he refused to use force again, and he would not let the hardliners continue a crackdown.

While all this was happening, some members of the KGB and Communist Party were taking advantage of the collapsing Soviet economy and getting rich by leveraging their positions into the nascent private sector. If you had the authority to OK a deal, you had to really try to not make money.

Kryuchkov saw all this happening -- a political and economic collapse -- and he felt he had to act before the signing of a new Union Treaty in August 1991.

When the coup took place, the Soviet military was divided. Most military officers remained silent or said they would obey lawful orders. A few vocally sided with Yeltsin, who rapidly became the symbol of an anti-coup movement. These included a paratroop training academy and garrisons on Sakhalin and the Kamchatka Peninsula. A few sided with the coup, most notably soldiers in Moscow.

Among the KGB, the linchpin during the coup was the Alpha Group, the KGB's prized commandos. During the failed intervention in Lithuania, they had been hung out to dry by the government, and when the coup came, many were hardly in a mood to cooperate: "The seizure of the television facilities in Vilinus eight months earlier had been the last straw. The politicians who had ordered the Alpha Group into action against a crowd of unarmed civilians had refused to assume any responsibility for the resulting bloodshed. They had even tried to disown the young Alpha lieutenant who was killed during the operation."

Alpha Group remained in its barracks during the night of violence outside the Russian White House. Some Spetznaz soldiers did move into the area in battle formation. Some KGB soldiers were in position around the building's exits.

On the night of August 23, 1991, rioters nearly attacked KGB headquarters but were dissuaded by armed guards and quick actions by Moscow city authorities and Gorbachev, who diverted their attention to the Central Committee building.

After the coup and amid the emotional riots, Dobbs concludes: "Nearly seventy-four years after seizing power in an armed insurrection, the Soviet Communist Party had ceased to exist. At the time of its demise it had fifteen million members. Not a single one of them put up any resistance."