Having read the entire Dark Towerseries, I felt it necessary to read this book, since it seems to anticipate much of what King writes later in the Dark Tower books.
The sense of imitation is eerie at times, and I found myself wondering more than once what Straub's other writing is like (since I've never read anything by him) because the novel felt overwhelmingly King-ish. It is a similar story to Roland's, though the hero in this story is a twelve year old boy (he turns thirteen on the final day). Jack and his mother are escaping his evil 'uncle' and he discovers he must go on a quest to retrieve the talisman of the title in order to save his mother (who is dying of cancer). Most people in the story have a 'twinner' in the 'Territories', another world that is similar to ours and yet much simpler, less poluted, and much more medieval. He 'flips' for world to world as he travels from New Hampshire to California, has many adventures along the way (including befriending a werewolf and being locked up in an 'orphanage'), retrieves the sphere, defeats the bad guy, and saves his mother. The story itself isn't a parallel for the Dark Tower series, but the idea of alternate (and connected) worlds is.
The fact that this book was written part way through the sequence of the first three Dark Tower books leads me to suspect that is was an exercise in which King tries to work out the ideas behind the Dark Tower's alternate worlds (something which I think he does more skillfully in the Dark Tower series than here, but hey, he's working out the kinks, right?)
I wouldn't necessarily recommend it unless you're a big King fan, or a big Dark Tower fan. I don't think it explained the Dark Tower any better than I had already gotten from the series.