It has to be LGG's A Scot's Quair - essential reading for all, can't be put down. Totally bawled!
Ring of Bright Water by Gavin Maxwell (and 2 sequels)
Para Handy by Neil Munro (and stop to visit the lovely Vital Spark if you pass through Inveraray)
The Prime of Miss Jean Brodie by M. Spark
Morven Caller by Alan Warner is good, Oban but a lot set in Spain too.
The Crow Road is wonderful (Banks writes novels as poetry, every sentence is pure craft).
Kidnapped and Catriona - classic R.L. Stevenson
Ten Tales Tall and True - or anything by Alasdair Gray (total polymath and lovely guy)
All Fun and Games until Somebody Loses an Eye is my favourite C. Brookmyre but it isn't set in the highlands.
The Sunshine Cruise Company by John Niven is hilarious but not set in highlands.
Consider the Lilies by Iain Crichton Smith is about the clearances so very sad but educational.
The Life and Death of St Kilda by Tom Steel also educational.
Listen to Scottish Fiction by Idlewild while you're adventuring
bonus points for reading some of Edwin Morgan's poems.
There are loads of great folklore anthologies if you are really interested in hearing the tales of selkie, morrigan, kelpie, etc - might give you the heebie jeebs but...