So studying Soviet intelligence operations can be a bit of a challenge because, as opposed to the United States, though Russian government has been very cagey and sparse in releasing its former intelligence files. There's a very good quote from Robert Pringle about this specific issue:

Analysis of Soviet and post-Soviet intelligence services is bedeviled by the problem of sourcing... historians and archivists busy trying to decipher the past from limited and contradictory material, always aware of the limits of the evidence.

Anyway, keeping that in mind, what do we really know? First, the Soviets were very very good at counterintelligence work. The area behind the "Iron Curtain" was almost universally considered a no-go area among Western intelligence agencies when it came to deploying agents. This is part of the reason why the US developed so much advanced surveillance equipment during the Cold War - it was the only way they could really spy on the Soviet interior.

Along the same vein, the Soviets were excellent at human intelligence (or HumInt). That is actually recruiting agents in the West or turning agents of Western intelligence agencies and gathering information through them. These agents were usually turned through a number of means, namely: blackmail, bribery, sex, or, in many cases, pure ideological loyalty to the communist cause. There were a string of high-profile individuals who were turned by the Soviets, scoring them major intelligence coups. The most famous is probably Kim Philby and the Cambridge Five, a group of five British spies who funneled tons of high value information to the Soviets. Philby in particular was incredibly valuable because he was eventually appointed the chief intelligence liaison between British intelligence and United States intelligence, which meant he could share information from both groups with his Soviet handlers. There were others as well. Aldrich Ames, Klaus Fuchs (who passed valuable atomic bomb information to the Soviets), Elizabeth Bentley (who was running rings involving hundreds of spies in the US), Rudolf Abel, and more. The Soviets really benefited from their HumInt and from their widespread assets across the West. It's honestly the defining aspect of Russian intelligence doctrine to this day. This in turn meant that the Soviets were quite good at assassinations. With assets everywhere, it was very easy for them to deploy quickly and rapidly to kill if they wanted to. The classic example is the assassination of Georgi Markov, a Bulgarian dissident living in London, with a poison pellet gun hidden in an umbrella. Assassination, however, was always a secondary goal to information gathering. When the Soviets wanted to know something, they always went to their human sources first.

As far as reading goes, again, the sources problem is very really and present. You may enjoy the following:

The Sword and the Shield: The Mitrokhin Archive and the Secret History of the KGB by Christopher Andrew, which is really one of the best books because Andrew got access to highly classified KGB files

My Five Cambridge Friends by Yuri Modin, which is a detailed look at the Cambridge Five and their motivations from the perspective of their handler Modin

KGB: The Secret Work of Soviet Secret Agents by John Barron, an outdated but interesting perspective on the KGB during the Cold War

Here's some other non-book sources on the matter:

The Theory and Practice of Soviet Intelligence

https://www.cia.gov/library/center-for-the-study-of-intelligence/kent-csi/vol7no2/html/v07i2a05p_0001.htm

Soviet Intelligence Training

https://www.cia.gov/library/center-for-the-study-of-intelligence/kent-csi/vol3no1/html/v03i1a08p_0001.htm

"Intelligence Services of Russia," by Robert Pringle in The Oxford Handbook of National Security Intelligence

Edit: Worth noting that, while the Soviets did maintain Signals and Geospatial intelligence capabilities, they were far inferior to those of the West because significant importance was not placed on them from within intelligence leadership. The Soviets usually had a lot easier time turning people in the West than the West did turning people in Russia, which made the necessity of technological development less important. They did, however, get quite good at hiding the information their spies were passing back in a variety of ways (in hollow coins, microdots, abandoned coke cans, or even using random chalk marks to indicate random drops in mailboxes). As a result of their counterespionage emphasis, they also got quite good at bugging and listening devices. The most famous case is known as The Thing, a powerful listening device and forerunner of RFID tech (and designed by the guy who invented the theremin) that was secretly installed in a carving of the US national seal that was given as a gift to the US ambassador to the Soviet Union. It remained, hanging in the office of the US ambassador, for almost six years before it was caught.