The iBook is excellent as you can listen the recordings with animated sonograms.
On the substance, I guess this is mostly interesting for birders, and maybe for those interested in animal “culture”, learned behaviors that spread across the population. At least in Finland, my feeling is that sound (at the sonogram level) is an under-appreciated aspect of birding.
This is a terrifically interesting book about all aspects of the sounds birds make, as it applies to birding: the problems with recordings, the use of sonograms, the variability of calls with age and sex, and so on. Specifically European birds, but the principles no doubt apply more widely.
I struggle with bird calls, perhaps because I don't have the ear for it, but mainly because I only started trying to learn them rather late and I don't spend enough time in the field putting it into practice. Maybe if this book had existed when I was 12 I would have ended up a much better birder… but that's probably wishful thinking.
Still, even if it doesn't make me a much better birder, I enjoyed it, it left me better informed, and the 2 CDs of birdsong are pretty good value in themselves.