We have lots and lots of children's books and this is one of my son's (and my!) favourites (we also bought 'The Pirates Next Door', which is excellent too).
The thing that sets this book apart are the illustrations - they are simply fantastic (detailed, bright, funny, colourful). There are also lots of hidden gems that mean you notice something new every time you read it (which helps if your son is like mine and wants back-to-back recitals of anything he likes)
The story is also written in a shanty style rhyme which adds to the pirate feeling (and also, I find, makes it easier and more interesting to read for the 20th time!).
Of the kids books we own (Night Monkey, Day Monkey - The Pirates Next Door - The Pirate Cruncher - Sir Scallywag and the Golden Underpants - What The Ladybird Heard - Jack and the Flumflum Tree - Charlie's Superhero Underpants - Cave Baby - Charlie Cook's Favourite Book - The Smartest Giant in Town - The Gruffalo's Child - Room on the Broom - The Snail and the Whale - Stick Man - Zog) this one, along with The Pirates Next Door and and 'Sir Scallywag and the Golden Underpants', are by far the best - if you own any of the others mentioned I thoroughly recommend you buy these for your children (and your sanity!).