With the popularity of MMA, there is no shortage of self defense advice - lot of it coming from practitioners of newer martial arts, like Brazilian Jiu-Jitsu. While that is a great choice to study for most people, the issue is that it takes a great deal of time and discipline to become proficient at it.
When it comes to preparing oneself for a violent street encounter, grappling or going to the ground (ie, the street) might not be the wisest choice for a number of reasons. Same goes for learning complex small joint holds, like finger or elbow locks, because with the huge adrenaline dump that goes with being attacked suddenly, your fine motor skills deteriorate very rapidly. (see Bruce Siddle's excellent book called "Sharpening the Warrior's Edge" for more detail about this).
What does work best in short violent street encounters are gross motor skill movements (knees, headbutts, elbows, etc) which direct maximum force against your adversary's weakest areas. There are no better manuals of study than these listed below. (yes they are old school, but definitely not outdated! The links to the books below are in pdf format)
Free Self Defense Books
"Championship Fighting" by Jack Dempsey. Dempsey was perhaps one of the top three boxers of all time, and his manual from 1950 is years ahead of its time. It was reproduced much later on by an author writing under the pseudonym "Ned Beaumont", who wrote a book in the late 90s called "Championship Streetfighting" which borrowed large sections of Dempsey's work. It is also common knowledge that Dempsey was also one of Bruce Lee's favorite fighters of all time.
Dempsey, a smaller individual, used superior technique to devastate many larger opponents. The "Manassa Mauler" used to delight in beating up big guys! This is a must read for any boxing-history buff or serious student of martial arts. It is the best book on boxing ever written! Print copies of this book are rare.
"Kill or Get Killed" by Rex Applegate. The late, great Applegate trained allied special forces in close-quarter combat during World War II. Kill or Get Killed was published by the US Marine Corps as Fleet Marine Force Reference Publication 12-80 with the introduction, "This reference publication was written in 1976 by Lieutenant Colonel Rex Applegate, USA (Ret), with the help of the Combat Section, Military Intelligence Training Center, Camp Ritchie, Maryland.
At last there is one volume which speaks to the subjects of unarmed combat (offensive and defensive), combat use of weapons, disarming the enemy, handling of prisoners, the handle of mob/crowd disobedience, the use of chemicals in such situations, and how to establish a professional riot control unit." Kill or Get Killed features unbeatable close-quarter combat techniques that will enable you to devastate any punks who would dare to attack you!
"Get Tough" by William E Fairbairn. Fairbairn was a British soldier, and police officer who was for most of his early career stationed in the former British territory of Shanghai, where he was forced into kill-or-be-killed street fight on a regular basis. His relatively short book encompasses all of the basic, easy to learn and apply gross-motor self defense skills - which are still the best to this day.
"The Secrets of Street Self Defense" by Paul Wellard. This is a relatively obscure, but thorough treatise on self defense from the 1970s. It was largely reproduced decades later by Peyton Quinn in "Secrets of Barroom Brawling", where large amounts of the text were reproduced. The author covers avoiding street encounters, the mental aspects of fighting and controlling fear, conditioning, takedowns, nerve strikes, grappling, improvised weapons and more. Like the other books mentioned, it is an old one, but "The Secrets of Street Self Defense" shouldn't be a secret!