All these things that I have done

I would say you should start with "Quite Ugly One Morning", but it's quite short and I know what it's like when you run out of things to read on holiday so instead I'll recommend One Fine Day in the Middle of the Night (http://www.brookmyre.co.uk/book4.htm) and if you can't find that one A Big Boy Did it and Ran Away (http://www.brookmyre.co.uk/book6.htm). Unless you're flying. Don't read The Sacred Art Of Stealing (http://www.brookmyre.co.uk/book7.htm) till you've read "A Big Boy..." though, it's not strictly a sequel but it might spoil the first book for you.