Here's a slightly edited version of my reply:
I wish a Christian would write a book on how to know when you've bought enough Christian self-help books. I think I have about 1,000 now. I'm including expositions on scripture, so I don't know if that would also qualify as self-help. Self-help sounds selfish but yet it is self that needs help because self-desire is the source of temptations.
I'm not sure if there is anything so confusing as knowing if answers to improving self can come from books apart from the bible. They are interesting to ponder over, but the huge problem that seems to arise from the whole principle is that it does not have the reader thinking of herself or himself less. How do you read a self-help book without thinking about yourself? I'm curious as to if and how you'd comment on that.
I think that for many reasons it's advisable not to think in terms of self-help though I understand the qualification you made. Likewise, I think it can be confusing to speak of self improvement. Both phrases, because of their pop-psychological association, give the impression that the self is autonomous and has the innate capacity to determine the ideal to which it should be moving and to effect the necessary change. I think it better to speak of maturing in Christ, growing in holiness, conforming to the likeness of Christ, etc. In other words, the language of progressive sanctification. This puts things in a gospel-centered context which keeps us focused on the grace of God and His goal - that we more accurately reflect Christ who is the true image of God.
Catchy, memorable sayings can sometimes prove helpful but more often than not I think they make things more cloudy. An example is the saying you mentioned: "Humility is not thinking less of yourself, it's thinking of yourself less." Thinking of myself less can mean one of at least two things. Usually, when we say that someone is always thinking of himself, we mean that he is in the habit of seeking pride of place. He selfishly seeks to get ahead, make a name for himself, and enjoy comfort and ease even at the expense of others. His satisfaction trumps all other considerations. In this sense, to think less of oneself would be to heed Paul's command in Philippians 2:3-4: "Do nothing from rivalry or conceit but in humility count others as more significant than yourselves. Let each of you look not only to his own interests, but also to the interests of others." As he goes on to explain, Christ is the exemplar of such humility in his willingness to obey his Father and serve us (despite his rightful claim to glory)even to the point of a shameful death. In light of this, thinking of myself less means being increasingly motivated by love for God and neighbor as opposed to exclusive self interest that makes the fulfillment of my desires the ultimate end for which I'm living.
Thinking of myself less could also mean that I have fewer episodes of self awareness. Humility, from this perspective, would manifest itself in my not having any consciousness of myself whatsoever. From your question about how you can read a self-help book without thinking about yourself, I take it that this is the sense you're assuming. But I don't think this is biblical let alone possible. It seems to have much more in common with Eastern philosophy in which the aim is to be absorbed into the impersonal One than with biblical Christianity.
God created us as self-conscious beings so thinking about ourselves is not inherently wrong. In fact, the Bible teaches us how we are to think about ourselves in relationship to God, others, and the rest of creation. Thinking about myself is a prerequisite to my obeying Jesus' command to treat others as I would have them treat me. I can't think of anywhere in the Bible that a complete void of thoughts about oneself is presented as a commendable goal. The critical issue is not one of the frequency with which I think about myself but that when I am thinking about myself it is with sober judgment (Romans 12:3)and not the exaggerated self-importance to which I am prone. The only way this will be accomplished is by my mind being renewed and reoriented by God's Word.
Since part of Christ's plan for building his church involves endowing some with the gift of teaching, I do believe that we can prosper from books other than the Bible. (Odd that people never question the value of sermons. It's always books for some reason.) to the extent that they help us understand and apply biblical truth.