Story involving teenager being recruited to travel to different worlds

Question

I read this story about five years ago (so make it about 2009) and the book (from the library) was reasonably tattered, but I'd say only 5 to 10 years old (this is just a rough guess).
This was a novel, I think about 300 pages, but again I don't recall it very well so that could also be completely wrong. I do remember it wasn't a huge tome though.

Most importantly: the narrative. This story involved a teenage boy who was recruited by a man on a motorbike to help in some expedition; I think they were part of a group involved in some sort of missions against an evil group (again I can't recall exactly what). This man (who I think was his Uncle 'Bob' or something like that) took him to a subway station and they only just made it to a portal to another world. I think I'm missing a huge portion of the story, but at the end I recall they ended up in another world (through a portal of some description). It was an ice world with a pre-Industrial civilization.

The key part of this novel that was members of this group trying to thwart the antagonists could only use a dog whistle; nothing else from a different world/time period could be used in that place. This was emphasized regularly and when the main character went to the ice world, he got into considerable grief with his mentor (the Uncle Bob character) for using a torch from his original time period.

I think it was set about the early 2000s. One last piece of information I recall was that this guy was regularly going on about three girls who were perfect at everything they touched. He had a crush on one called Courtney.

Sorry for the vague details at the beginning, but hopefully the narrative description could prove useful. Thanks ;)

Answer 1

This is "The Merchant of Death" by D.J. MacHale, the first book of the Pendragon/Halla series.

The book revolves around a young boy named Bobby Pendragon whose uncle takes him (and his best friend Mark and his major crush, Courtney Chedwynde) into a magical world called Denduron.

His uncle gives him a whistle to help ward off local animals known as quigs but he ends up using a flashlight from home as a supplementary weapon, against his uncle's advice

This may be the last journal I write to you, Mark and Courtney. If it is, then please know that it wasn’t your fault about the flashlight. All you did was help out a friend. The blame is all mine. If you don’t hear from me again, then please know I did everything I could to undo the mess I created. I may not be successful, but at least I tried. Thank you for reading this, and for being my friends.