Erak's ransom (Ranger's Apprentice book 7)
It was a friend of mine - an adult, and not working/associated with kids or teens in any way - who told me about the Ranger's Apprentice books just after the first one came out.
They are great reads - sometimes a bit too much action, and I find myself skimming through just because I need to know right NOW that all will be ok. The characters are great.
I did enjoy this book, but find it slightly annoying in that it goes back to a period of time BEFORE the last two books...
Books 1 to 4 dealt with Will's time as a Ranger's Apprentice, 5 and 6 Will as a fully-fledged Ranger. And then, book 7, set just before Will graduates from apprentice level.
Apart from that - it's always nice catching up with characters you enjoy visiting.
For more info, check out the two websites - the original, Australian one (which annoyed me with the install/don't install flash message EVERY time I visited a page!) and the international one.