What's the best way to train using books, DVD's or online training?
The key is finding at least one compatible training partner who will gradually orchestrate attacks at full force and resistance to the counter-tactics. Women need to train with men and men need to train with men. Obviously, no one should get injured in any type of training. Yet, training cannot be done in a vacuum devoid of the brutal nature of an attack, especially, if it is an ambush. The balancing act (literally and figuratively) is safety.
A method to prepare mentally -- other than harvesting real-life experience -- is to do an internet search for "real street attacks." Viewing real-life violent encounters partially allows the viewer to witness the ferocity, no-holds barred nature of an attack. This supports and undergirds krav maga's core tenet that there are no rules in a fight for an attacker or defender.
The the best way to learn remotely is to find a good tough workout partner. Go through each technique on the DVDs with each person repeating it. For the combatives, be critical of one another's movements to make sure the hands are up, the hips are pivoting correctly, the movements are long and crisp along with not telegraphing them. I would recommend going through each technique in the book/dvd and practicing it a minimum of 12-15 repetitions per partner.
For any technique, always try to simultaneously (or near simultaneously) defend and attack together. Also, try to always make the movements long even though even while being careful not to strike your partner. For retzev, I would suggest practicing in front of a full-length mirror to observe the fluidity of your movements, your pivoting/weight transfer, and balance.
In addition, you can practice the combatives in front of a mirror. If you are going to practice the combatives in the air against your partner take a step back so you can execute almost the entire movement without locking your elbows or knees. In other words, try to avoid short movements. To develop long movements along with power you might want to invest in a 120 lb heavy bag or some good muy thai pads and a solid kicking shield.
For all defenses (including weapons), you may wish to practice the techniques from a neutral or hands down (-5) position to simulate an ambush. These defenses are obviously more difficult because you are unprepared (even though obviously you know something is coming because you are training.) Have your partner withdraw the attacking limb as an aggressor will continue the attack or try to protect/withdraw his weapon to use it.