Evidently this book was written by a Scottish nationalist who abhors any connection to the English. Some of early peoples of Scotland were the Picts, Gaels, and Angles. 'Angles' isn't used by this author --note the title--and is replaced by 'Scots'.
The Scoti were a Gaelic speaking people who migrated from Ireland in the first centuries of the Christian era. In Latin the Gaels were called Scoti. In the Middle Ages their language, Gaelic, was the language of most Scots
although a half dozen languages were spoken in Scotland.
The Angles were a Germanic people who invaded what is now the north of England and established the Kingdom of Northumbria. In the seventh century they invaded Lothian, the Edinburgh region. Their language, variously called Anglian, Inglishe or English, was adopted by the Scottish monarchs, and spread with the establishment of burghs through much of Scotland.
In the modern period English-speaking Scots began to call their language 'Scots'. Some historians and archaeologists now use the term 'Scots' for Angles; not only is it anachronistic, it is misleading.
Better books for the early history of Scotland are:
Stephen Driscoll, Alba: The Gaelic Kingdom of Scotland AD 800-1124
E.J.Cowan & R.A. McDonald, Alba: Celtic Scotland in the Medieval Era
W.A. Cummins, The Picts and their Symbols
Anna Ritchie, Picts: An Introduction to the Life...
Alfred P. Smyth, Warlords and Holy Men: Scotland AD 80-1000