My guess is that purists would insist it has to be whisky in a toddy, but I feel that brandy or rum are perfectly acceptable substitutes according to taste and available ingredients. I always have more whisky around than anything else (apart from gin, there's quite a bit of gin in my flat too) so always use it, and it's always a single malt because again that's what I have. I like to keep it simple - a generous splash of whisky in the bottom of a mug, an equally generous slice of lemon given a bit of a squeeze, a spoon of honey to taste. I was looking up recipes before writing this post and am slightly, as well as irrationally, sniffy about how elaborate some of the recipes are (all-spice and juniper are all very well, as indeed is a cinnamon stick, but I'd argue that's moving towards a punch). It's clearly my deep held belief that a toddy should be confined to 3 good things and some almost boiling water.
The book is obvious - Whisky Galore by Compton Mackenzie is a treasure. Funny, charming, sharp, and essentially affectionate as well as owing a debt to an actual incident it has a lot to recommend it. The film is just as good and something of a pre Christmas tradition for me. Watching it with some sort of whisky based drink in hand is surely essential - for authenticity it would be a blend (I'll confess that spotting the brands is an essential part of watching the film for me) because that's what we drank before Glenfiddich really launched the single malt market in the 1970's. It would also be neat spirit in the kind of little tot glasses rarely seen outside of charity shops these days - but a toddy better captures the spirit of both book and film, and something warm in a mug that won't knock you out is - well, it's civilised.