Read any good books lately?

The crucial part of a musical is the music, right? Wrong. It isn't the lyrics either. The secret of success lies in the book.

Before you head for the bookshelves, let me be clear. The book of a musical is not a novel, it's the spine of a show, the dramatic skeleton upon which musical flesh sits. It's the essence, utterly crucial and almost always unnoticed.

Before you head for the bookshelves, let me be clear. The book of a musical is not a novel, it's the spine of a show, the dramatic skeleton upon which musical flesh sits. It's the essence, utterly crucial and almost always unnoticed.

In Broadway's heyday when dozens of musicals opened during every season, there were awards for Best Book alongside those for Best Musical and Best Score. Quaint though it may seem, back then there was more to a good show than a big budget, over-amplified sound and a set on hydraulics. There was a recognition that the form was theatrical, and that didn't just mean that it took place in a theatre. Not for nothing did Broadway's legendary producer/director, George Abbott, claim that the three things you had to get right in a musical were "the book, the book and the book".

It may sound fatuous to say it, but musicals should be dramatic. Unless you're glued to what's happening on stage, chances are you'll become disengaged and bored, so why on earth would you register or remember anything but the most repeated of tunes? Outside of the new breed of brazen crowd-pleasing hits compilations - Buddy, Mamma Mia - or the shameless staging of an audience's collective movie memories - Saturday Night Fever, Footloose - the secret of success lies in a good story.

The advent of the through-sung musical - curtain up, everyone sings without stopping for two hours then curtain down - meant that for about 20 years people stopped constructing shows with clearly defined roles for music and/or drama. Instead you had a kind of wash in which every bit of dialogue was sung.

But the signs are that taste is swinging back towards the more traditional "musical play" as evidenced by the reception accorded to the latest revival of Jesus Christ Superstar, which opened on Broadway last Sunday night to reviews the wrong side of hostile. The score, it seems, is no longer enough.

What chance then a new dialogue-free show like Tess? For reasons best known to its backer - a man with no previous theatre experience - this disaster (mal)lingered for several weeks in the hope of riding out disastrous reviews. But audiences weren't fooled. A good novel doesn't necessarily make for a good book.

The same applies to real life. Take Lautrec. This heads back towards the division between songs and scenes but its artistic failure proves just how difficult a balancing act it is. The artist's life hardly lacks theatrical potential, but a gallery of episodic scenes certainly isn't the same thing as convincing an audience through strong theatrical storytelling.

Even Cameron Mackintosh is moving back towards the book show with the upcoming The Witches of Eastwick, its creators, John Dempsey and Dana Rowe, having already proved conclusively with their flawed but fascinating The Fix that they know how to make real musical drama. They have clearly taken a long hard look at the great books of yesteryear, something producers would be well advised to do.

The two contenders battling it out for the prize of Best Book Ever are Gypsy and The King and I. Contrary to the popular misconception that all musicals are footling and fluffy, these shows are vivid dramatisations of complex ideas. They each have great scores but the reason that they have lived on as drama is because they're actually about something.

Gypsy, ostensibly the life of burlesque queen (ie stripper) Gypsy Rose Lee, is really an exhilarating but excoriating portrait of the horrors of brute selfishness. The King and I, an adaptation of the film of Margaret Landon's chronicle of Anna Leonowens and King Mongkut of Siam in the 1840s, may have been a love story of sorts (the nearest the title characters get to a kiss is the climactic "Shall We Dance") but it is really about the clash between and politics and personalities, the struggle between authority and freedom.

Rodgers and Hammerstein were no strangers to big ideas. Their South Pacific, widely sneered at for its lush exoticism is, among other things, very obviously an attack on racism. But The King and I is a more completely rounded piece. That an American musical without a single American character should have proved so enduringly popular is a testament to its masterly construction.

It began as a star vehicle for Gertrude Lawrence but by the opening on 29 March 1951 it had developed into an extraordinarily subtle piece. A great production of it reveals the truth: the best musicals aren't about songs, they're about a uniquely dramatic combination of music, lyrics... and book.

'The King and I' is at the London Palladium and opens on 3 May (020-7494 5020)