The biography uses mostly 'secondary' sources. [=information that did not come from original/primary sources but came from research done by others]
There are some 'secondary' issues/considerations which must be taken into account as well.
In some children's books, the text is 'secondary to' the illustrations.
(..) I'd wager the contents are rather more incendiary. Who is this? Hello, darling. Oh dear... I told you not to move it. It seems a secondary charge has been activated. Sweet thing, I might need your help in the disposal of this parcel. One hundred... One million pounds! Oh, And Um, By the way, Fire (..)2011 Sherlock Holmes: A Game of Shadows