By the Book

"The media and the government would have us believe that torture is some necessary thing. We need it to get information, to assert ourselves. Did we get any information out of you? Exactly. Torture's for the torturer...or for the guy giving orders to the torturer. You torture for the good times - we should all admit that. It's useless as a means of getting information."

- Trevor Philips

By the Book is a mission in Grand Theft Auto V, given by FIB agent Steve Haines to protagonists Michael De Santa and Trevor Philips.

Mission
Michael receives a phone call from Steve Haines telling Michael to go to a warehouse and meet Trevor. There, he and Trevor find Haines and Dave Norton along with Mr. K. Haines wants Michael to assassinate Tahir Javan, an Azerbaijani man who Haines and Norton claim has ties to terrorism. Mr. K previously set up an audio-visual system for Javan and so is the FIB's sole source of information. Haines orders Michael to go with Norton to locate Javan, while Trevor and Haines remain in the warehouse with Mr. K.

Michael and Norton drive to their first location, a home in Los Santos, but this turns out to be a false lead. Haines then orders Trevor to torture Mr. K in order to discover Javan's whereabouts. There are four ways in which Trevor can torture Mr K: (Mr. K can't be pushed too far. If this happens, he'll need an adrenaline shot, which requires injecting him with a syringe. Mr K can die up to three times before Trevor runs out of adrenaline) The first round of torture reveals that Javan is actually at a beach house in Chumash. Michael and Norton drive to the home, but once there, they discover that a party is in full swing and that there are several men present who could possibly be Javan. At this point, Haines orders Trevor to engage in more torture. Through three additional rounds of torture, Mr. K reveals that Javan is: Through these clues, Michael is able to identify Javan and assassinate him.
 * Waterboarding
 * Beating him with a wrench. Where Trevor is when the player presses the button depends where Mr. K is hit. They have the same effect and damage. Mr. K's left = knee hit. Middle = "ballbreaker". Right = ribs.
 * Electrical shocks from a car battery
 * Extracting a tooth using pliers. First tooth changes speech. Unknown if more pulls either further deteriorate his speech or nothing at all.
 * A man of average height and weight (Able to kill Javan at this point to complete the mission early.)
 * Has a beard
 * Is a chain smoker and left handed

Back at the warehouse, Haines tells Trevor that Mr. K is expendable and orders Trevor to kill Mr. K after Haines has left. Once Haines departs, Trevor instead decides to free Mr. K. He guides Mr. K from the warehouse and into a waiting vehicle, at which point he drives Mr. K to the airport.

Gold Medal Objectives

 * Don't Stop Me Now - Complete without Mr. K's heart stopping.
 * Electrocutioner - Electrocute Mr. K.
 * The Tooth Hurts - Pull out Mr. K's tooth.
 * Wrenched - Hit Mr. K with the wrench.
 * It's Legal! - Waterboard Mr. K.

Trivia

 * This mission is said to be the one of the most controversial content in the whole franchise, and have received heavy criticism by critics from various media outlets.
 * The sub-achievement "Don't Stop Me Now" is a reference of a popular song by Queen, which has the same title.
 * This mission can be triggered by Michael or Trevor.
 * If the player begins this mission as Michael, before entering the warehouse, Trevor can be seen defecating behind a dumpster, much to Michael's disgust.
 * If the player begins this mission as Trevor, before entering the warehouse, Michael will be seen smoking and Trevor tells him to quit that and shape up.
 * This also may cause a bug. If the player is wearing a full outfit which includes shoes, Trevor's model will switch to his default jeans, but not default boots. This makes him have no feet.
 * It is possible to assassinate Javan early (not getting all the information) successfully, to cut the torture session short. This does not change the outcome of Mr. K's condition for the most part, but some dialogue may change depending on what torture tools you didn't use. (i.e. No pliers = clear speech and different dialogue, etc.)
 * Also, while replaying the mission, if you kill Javan early, the game will rarely bug and Michael will stand still aiming, the player will not be able to switch to Trevor and there's no Misson Failed screen. The only solution is to pause and load a previous game file.
 * After the coffee break, a glitch may occur that causes the coffee cup to stick in Michael's hand.