For Robert Burns books and sheet music you can buy, check the Robert Burns Collection at Sheetmusic Plus
You may also be interested in other Scottish Music on this site:
Robert Burns, Collected Works Songs and Lyrics (700+) items with PDF versions
Scottish Sheet Music & Guitar Tabs, 4000+ tunes with Sheet Music, Guitar Tabs, midi files, and PDF
Scottish Sheet Music & Mandolin Tabs, 4000+ tunes with Sheet Music, Guitar Tabs, midi files, and PDF
Traditional Scottish music songs & tunes, 140+ favourite tunes & songs - midis, lyrics & sheet music
Robert Burn's songs, collection of 360+ items from Scotland's Bard with lyrics & sheet music.
Many Scottish tunes are included in the following Folk pages:
Folk sheet-music and guitar tabs, 10,000+ tunes with Scores, Guitar Tabs, midi and PDF
Folk sheet-music and mandolin tabs 10,000+ tunes with Scores, Mandolin Tabs, midi & PDF