Based on his doctoral thesis of the same name (Boston College 2004) this BAR report is scholarly with a full bibliography and detailed footnotes and references. It contains considerable amounts of valuable information, usually presented clearly, but needs to be used with care.
As the title states, the thesis was about castles built before 1100 although it does contain useful information about almost all the timber castle site in the region under study (The historic counties of Bedfordshire, Cambridgeshire, Huntingdonshire and Northamptonshire). As the archaeological dating of sites is rarely precise and the historical documentation rarely conclusive the dating given by Lowerre to some sites is bound to be contentious (Excluding the ringwork at Alderton even as a 'possible' 11th century castle seems perverse).
More importantly Gatehouse feels that, to some extent, Lowerre has a paradigm of the early castle which is limited and which does not appreciate the full diversity of the functions of the castle (He excludes Chalgrave as an 11th century castle despite the rare actual archaeological evidence that it is 11th century in date because the motte and bailey that existed there are too weak to have been a 'true' castle.)
Some of his comments on possible castle builders are based on estate sizes. Certainly castles did have a cost for construction but a wider survey of castle sites (particularly in the welsh marches) would have shown a good number of small castle built by relatively poor land holders who Lowerre may have wrongly excluded as possibly castle builders in his survey area (i.e. Biggleswade).
The book is, therefore, a useful resource for its bibliographic references and for much of its raw data but the interpretative conclusions drawn may be questioned and should be used with care.