Lewis writes about a variety of topics in the Psalms that strike him as significant. First, he notes the difference in the Psalms's presentation of divine judgment and the Christian's understanding of it. Christians usually think of the dies irae judgment when God separates the sheep from the goats or the wheat from the weeds at the end of time, an act full of fear and awe. The Psalms look at judgment as winning a civil lawsuit--the unjustly persecuted or deprived has their day in court and their recompense. The Psalms either praise this when it happens or implore God to make it happen. The contrast of views does not mean one is right and one is wrong; rather, a greater richness can be discovered in both views.
Other topics he looks at are death in the Psalms (which don't seem to embody the fully developed Christian notion of an afterlife), the sometimes shocking, sometimes juvenile cursing of enemies/desire for revenge on persecutors (even Psalm 23 wants God to set up a festive table in front of enemies, as if to rub their noses in it), the love for God as embodied in nature and in temple worship, the love of the law as something truly to embrace as joyful and not to fear as punishing, and several other ideas or themes.
The book is delightfully accessible. Lewis uses very down-to-earth language and explanations. It's as if he was talking with you rather than lecturing at you. He fosters a personal relationship through his writing. Isn't that what we as Christians look for in reading the Bible, to find a more personal relationship with God Himself? The book helps the reader be a better Christian by better knowing God through the Psalms.
Here's a sample where Lewis discusses the different interpretations of why certain Pagan myths are similar to the death and resurrection of Jesus Christ:
And what are we to say of those gods in various Pagan mythologies who are killed and rise again and who thereby renew or transform the life of their worshippers [sic] or of nature? The odd thing is that here those anthropologists who are most hostile to our faith would agree with many Christians in saying "The resemblance is not accidental". Of course the two parties would say this for different reasons. The anthropologists would mean: "All these superstitions have a common source in the mind and experience, especially the agricultural experience, of early man. Your myth of Christ is like the myth of Balder because it has the same origin. The likeness is a family likeness." The Christians would fall into two schools of thought. The early Fathers (or some of them), who believed that Paganism was nothing but the direct work of the Devil, would say: "The Devil has from the beginning tried to mislead humanity with lies. As all accomplished liars do, he makes his lies as like the truth as he can; provided they lead man astray on the main issue, the more closely they imitate truth the more effective they will be. That is why we call him God's Ape; he is always imitating God. The resemblance of Adonis to Christ is therefore not at all accidental; it is the resemblance we expect to find between a counterfeit and the original, between imitation pearls and pearls." Other Christians who think, as I do, that in mythology divine and diabolical and human elements (the desire for a good story), all play a part, would say: "It is not accidental. In the sequence of night and day, in the annual death and rebirth of crops, in the myths which these processes gave rise to, in the strong, if half-articulate, feeling (embodied in many Pagan 'Mysteries') that man himself must undergo some sort of death if he would truly live, there is already a likeness permitted by God to that truth on which all depends. The resemblance between these myths and the Christian truth is no more accidental than the resemblance between the sun and the sun's reflection in a pond, or that between a historical fact and the somewhat garbled version of it which lives in popular report, or between the trees and hills of the real world and the trees and hills in our dreams." Thus all three views alike would regard the "Pagan Christs" and the true Christ as things really related and would find the resemblance significant. [pp. 105-107]