Gerald Sinclair and Rondo B B Me's new book The Enigmatic Sinclairs is, in a word, extraordinary.Before its publication, researchers like me relied on books like:
R.W. Saint-Clair’s The Saint-Clairs of the Isles, being a History of the Sea-Kings of Orkney and their Scottish Successors of the Sirname of Sinclair - published in 1898
Thomas Sinclairs, The Sinclairs of England - published in 1887
Leonard Morrison's Sinclair Family - published in 1896
Each of these books is useful in its own way. Saint-Clairs of the Isles relied much on the work of Father Richard Augustus Hay who was a solid researcher. Sinclairs of England has some good information mixed with speculative, unsupported leaps. Sinclair Family picks up much of the same myth-making which is in the other two about Rollo the viking being our common ancestor and our Norse origins. That said, each is useful and I recommend them all as part of your research library.
Now we have something entirely new - years and years of newly researched facts written into an incredibly readable, thoroughly engrossing resource which I recommend every genealogist own and study - even if you're not a Sinclair, just to learn and then use Gerry and Rondo's methodology.