There are a handful of books that might be modern day YA classics; this is one. Such a statement almost demands a meta-rumination on the making of a classic, which I can't offer; what I can offer is an effort to identify what keeps me coming back to this text with at least annual regularity, and each time reveling in its lively prose, its whimsical characters, its charming, quirky narrative. 
 Most broadly, Howl's could be read as a kind of parable about overcoming life's difficulties, challenging one's own stereotypes, looking beneath the surface, etc--but that is not where the novel's most enduring qualities lie; it is within the writing itself. Jones's language throughout the novel sparkles with vivacity and humor; on a microcosmic level, this book is simply a delight to read. The images she evokes betray an extraordinary skill with English; she brings completely implausible scenarios to life, and manages to infuse them with a pervasive levity and dry observation of human foibles. For example, "Michael came bounding down the hillside after her, and the tall black castle came bobbling along behind him, shedding anxious puffs of smoke from all four turrets" 
 This single sentence illustrates the delightfully human, anthropomorphic objects that populate Howl's. It is little wonder that Miyazaki was inspired to transform it into a film (which, imho, falls very short of the richness of the text). 
 In my opinion, this is by far the best work that Diana Wynne Jones left us; it preceded the fantasy mania brought on by the Harry Potter franchise and is exponentially better in terms of writing quality.