Different books with 'discipline' in the title could be talking about entirely different things. Let's get our definition straight so that we're not disagreeing.
This is the first definition that appears on Google: "The practice of training people to obey rules or a code of behavior, using punishment to correct disobedience."
On a site like this, I think we're more interested in self-discipline. So, we'll redefine discipline in our discussion as this:
The practice of obeying rules or a code of behavior.
I'll also use the words
self-control interchangeably. I think that some literature might differ between the two, but I'll assume that they're the same.
Discipline is not simply willpower
I'll quote the abstract of a psychological study directly:
Exerting self-control may consume self-control strength, reducing the amount of strength available for subsequent self-control efforts. Coping with stress, regulating negative affect, and resisting temptations require self-control, and after such self-control efforts, subsequent attempts at self-control are more likely to fail. Continuous self-control efforts, such as vigilance, also degrade over time. 
You can read the article itself for deatils. But the important thing to take from it is that everyone starts with the same amount of willpower. If you've seen infants, babies, and children, they start with remarkably low self control - a 3 month old child will get excited to the point of crying. But they build it up over time.
An experiment was done. One group was asked to eat fresh cookies, the second group was shown the cookies, but asked to eat radishes instead. They were both given impossible puzzles to solve. The cookie group worked on the puzzles for 20 minutes on average, the radish group worked on it for 8 minutes. The radish group also became more hostile towards the experimenters.
The experiment found that the same resource was used for making decisions - someone who had to make difficult decisions had less willpower. It applies to being nice to mean people and doing things we don't want to do. They found direct correlation with blood glucose supply too.
However, they find that it does get stronger with exercise. Baumeister recommends practicing overrriding habits and exerting control over your actions. Use your left hand to brush your teeth instead of your right hand. Resist buying candy and cigarettes for a whole day or eat healthier. 
The most famous example of an extremely well-disciplined force is the military. How do they do this? By forming habits. Militaries are the greatest experiment in habit-formation. Militaries are tasked with turning uneducated, undisciplined people into a group that can follow orders exactly, without question. Every routine in a soldier's life is planned from morning to night - when they wake up, when they sleep, how they brush their teeth or maintain their gear.
Habits require no self-control to do. In fact, they do require self-control to break. 
From an outside perspective, this looks painful. But if you wake up every day at 5 AM, it becomes difficult to suddenly wake at 8 AM. If you're used to brushing your teeth and showering every morning, it suddenly becomes difficult not to do so.
These things all have a tiny cost in self-control. Once they're built into a routine, their cost is reduced to zero. If waking up and jogging costs no willpower, you have more willpower for more difficult tasks ahead.
Self-discipline vs external discipline
So why do we procrastinate? Why don't we wake up at 5 AM? Why do we eat unhealthy food?
Self-discipline isn't externally applied discipline. Imagine yourself sitting in front of a computer, doing work. How long does it take before you wander off on the Internet? Now imagine your boss sitting behind you. How long does it take then?
It's extremely easy to be 'disciplined' when someone is imposing it on you. But a lot harder when you're expect to discipline yourself. You can't simply tell your boss "I'll do this work after I watch this movie", but you'll tell it to yourself all the time. 
There's a lot of literature comparing ourselves to Jekyll and Hyde . We have two parts of ourselves wanting very different things. One side of us wants to quit smoking, the other wants to buy bigger packs of cigarettes, no matter what the price. One side wants to jog in the mornings, the other wants to sleep in. We'll call this 'dark' side the Hyde side.
Your inner Hyde is not evil - it just wants to have fun. It's the impulsive, happy-go-lucky, charming, creative side. It lives in the now, never thinking in the long term. If you were to completely supress this side, you'll become boring, uncreative, uninspired.
Most of us are trapped in an endless struggle with our Hydes. We choose to procrastinate studying for an exam. If we studied, we'd never be able to study enough. We'd be spending our lives studying harder to beat our previous grades. If we did our best, we couldn't force denial on ourselves; there's no way to cope with the disappointment if you did your best and couldn't pass. It's easier to tell yourself you failed because you didn't do your best. In many jobs, people are punished for working too hard - the faster they get things done, the more their bosses expect them to get things done faster. 
There's a long list of CEOs and company founders who wake up at 4.30 in the morning to exercise. How do they do this?
A good number of them also live by a "no excuses" lifestyle. There is no way for them to justify that they're not doing their best. Their jobs require them to perform extremely well or get fired.
Aside from habit, many of them are entrepreneurs. They have a vision in mind, which their Hyde side also enjoys. If you've ever started a business, there's a certain kind of thrill felt when everything you do brings results. You stop working against yourself and even your negative side starts dragging you forward.
 Self-regulation and depletion of limited resources: Does self-control resemble a muscle?, Muraven, Mark; Baumeister, Roy F., 2000.
 Ego Depletion: Is the Active Self a Limited Resource?, Roy E Baumeister, Ellen Bratslavsky, Mark Muraven, and Dianne M. Tice, 1998.
 Thinking, Fast and Slow, Daniel Kahneman (Nobel Prize winner), 2013.
 The Power of Habit: Why We Do What We Do in Life and Business, Charles Duhigg, 2013.
 Self-Discipline in 10 days: How To Go From Thinking to Doing, Theodore Bryant, 2011.
 Predictably Irrational: The Hidden Forces That Shape Our Decisions, Dan Ariely, 2010.