The Abernethys of Saltoun and lineage thereof can be obtained from Sir James Balfour Paul's 9 volume work, "The Scots Peerage", published in Edinburgh between 1904 & 1914, available on microfilm via the Church of Latter-Day Saints Library, LDS British microfilm call numbers 104157-104161.
Sir James Balfour Paul was the Lord Lyon King of Arms for Scotland at the time. This is the penultimate and official authority of the British Government on all matters of Scottish Aristocracy. No more authoritative source exists for Scotland's titled families than the Lord Lyon King of Arms.
There was no William 9th of the Abernethy (or Abernathy) line prior to the granting of the title of Baron. The 1st Abernethy Baron of Saltoun and _8th_ of the line was born about 1410 and d. 13 May 1460/61.