There is some research into the efficacy of self-help versus therapy. Usually these studies find that self-help is less effective or not effective at all (e.g. Ehlers et al., 2003, to give just one example), although the relative effectiveness depends on the type and severity of disorders. For example, fear of flying is probably easier to self-treat than a personality disorder like antisocial behavior. The problem with this research in the context of your question is, that the self-help in these studies does not consist in handing out CBT manuals to the patients, but "self-help booklets", and that of course these are limited in content when compared to the full knowledge of a therapist. So to answer your question, I will assume that the "patient" in question knows as much about CBT as any therapist would. Because that would be the prerequisite to applying CBT to him- or herself.
In my opinion there are several problems with self-therapy:
The first step in any therapy is a careful diagnosis. You want to know what your problem is, before you start working on something that you don't suffer from. E.g. you might think that you have a depression, when in truth you are suffering from something that also has depressive symptoms, e.g. social phobia or an eating disorder. Diagnostics is a science of its own and psychologists spend quite some time during their education to learn it. I don't think a layperson can reliably diagnose a psychological problem.
Even if you are an educated psychologist yourself, an honest self-diagnosis might be impossible. Most people – and psychologists are just normal people in this – don't perceive themselves clearly, but are influenced by shame or misconceptions. A typical example is an eating disorder: the whole problem with most eating disorders is that the patients are unable to understand that they have a completely faulty image of themselves. They believe they are overwheight, while they are in fact completely normal or even underwheight. Such a person would never diagnose herself with an eating disorder. Most other mental disorders are the same: the concerned persons don't perceive their problems as problems, or not accurately.
Let's assume you have an accurate diagnosis. Maybe a psychologists diagnosed you, or you are uncommonly objective in your self-assessment (although most likely you fool yourself here, but let's just assume you know what you have). There is the problem that even therapists don't usually know what a patient has at the beginning of a therapy, or not definitely or completely. Their diagnoses are given provisionally, and adapted to new insights they may gain during therapy.
Okay, but let's assume you really, really know what your problem is. Couldn't you help yourself? To answer that, we must understand what a psychological disorder is. There is this model that thinks of all human behavior, normal as well as deviant, as an adaptation to a certain surrounding. You are being abused by your parents, and you develop dissociation to deal with it. This strategy of survival persists even after the situation that caused it no longer exists, because it was a successful strategy and because either it still works (to some degree) or you find yourself unable to unlearn it. There is something inside yourself that keeps you applying the same solutions to the same problems over and over again, even though other solutions would be better. There might be an underlying fear, or simply a deeply ingrained learning, whatever it is, you will find yourself faced with a very strong inner resistance against changing your behavior. What the therapist does is he adapts the therapeutic strategy to your resistance. He observes how you react to the therapy and finds just the right amount of therapy to slip in under your resistance. He finds the kind of things you find yourself able to do. And he offers his emotional support. In a therapy you are not alone, there is someone you can fall back to, someone who understands, accepts and appreciates you as you are, and at the same time supports your attempts at change. It is more than just doing gymnastics exercises blindly each morning. If CBT was only a number of exercises, there would be no therapists, only researchers that published these books with exercises. But these exercises lack adaptability and the human interaction that breaks your resistance to change.
Okay, but what if I know my problem and I'm not self-delusional and I'm really able to deal with my inner resistance? Well, chances are that you don't have a psychological disorder anyway. If your fears and habits don't keep you from dealing with your problems, then you don't have problems. Or only because you don't suffer enough from them to take care of them.
All disorders show a certain percentage of what is called "spontaneous remissions": the problems disappear without therapy. I'm pretty sure that in some of these cases the persons dealt with their problems themselves in some way. They got over it. Not everyone remains hampered for life after child abuse. Some people look at that experience, limit it to the people who abused them, and have normal and healthy relationships with everyone else. Somehow some people get out of their disorders after some period of "darkness" and are able to lead a normal happy life without the help of a therapist. Probably reading a CBT manual and applying some of the insight to one's own life will help some people make a change on their own, and it very likely won't hurt any of the others, but most likely it was something in that person, a certain resilience, or something in their surroundings, like the support of friends or familiy members, or a fundamental outside change, like moving to a new city, that effected this self-healing, not a CBT manual.
- Ehlers, A., Clark, D. M., Hackmann, A., McManus, F., Fennell, M., Herbert, C., & Mayou, R. (2003). A randomized controlled trial of cognitive therapy, a self-help booklet, and repeated assessments as early interventions for posttraumatic stress disorder. Archives of General Psychiatry, 60(10), 1024-1032.