The poem's playful alliteration adds to its comic tone, which adds a layer of irony or self-parody to the speaker's narrative. The first example comes in lines 5-6. When the speaker was a kid, books offered him an escape from reality through the imagination. In this imagined world, he could:
[...] deal out the old right hook
To dirty dogs twice my size.
The /d/ sounds are plosive, meaning they require the expulsion of air when spoken. Poets often use plosives when they want to convey something physically powerful or violent, and that effect is definitely going on here (the lines are describing a fistfight). But there's also something cartoonish and child-like about the phrasing here; it's the kind of vocabulary that the speaker might have encountered in an adventure novel or a comic book.
Lines 13-14 use the same sound, only now it describes how the speaker has given up on the escapism of books:
Don't read much now: the dude
Who lets the girl down before [...]
Here, the /d/s awkwardly echo the more enthusiastic lines 5-6, as though those early books still reverberate in the speaker's speech patterns. The insistent consonants—which link a series of negative words ("Don't," the mocking "dude," and "down")—sound deflated, like a litany of failure.
In lines 16-17, the speaker says that the "dude" and the coward who:
[...] keeps the store
Seem far too familiar. Get stewed:
These /s/, /st/, and /f/ sounds have a spitting, almost venomous quality. The alliteration thus captures the speaker's self-loathing, and his resentful belief that "Books are a load of crap."