This book builds on Laycock's earlier "Britannia: the Failed State" which examined the evidence (mostly from archaeology) for the disunity of Britain before, during, and after the Roman occupation. "Warlords" concentrates on the last period, and in particular on certain leaders, combining the archaeology with written sources for post-Roman Britain. "Warlords" certainly has ideas and evidence worth thinking about. However, it is a less scholarly work than "Britannia", and Laycock goes further into the territory of speculation.
Laycock's central thesis is that there was no national Brittonic identity, but rather only tribal affiliations, and that there was no authority recognized above that of local kingdoms, based around one, or sometimes two, tribal areas / civitates. Ultimately, I was not convinced by his arguments. For example, the existence of memorial stones that name people by their tribe does not prove that there was no sense of Brittonic nationhood. All the relevant contemporary writers I can think of (Constantius, Gildas, Sidonius, Jordanes, Procopius) identify the Britons as a nation and many imply leadership of that nation by an individual (the proud tyrant, Ambrosius, Riothamus, an unnamed king) at least at certain times. Laycock's attempt to localize these individuals to particular tribes is hard to reconcile with what these authors wrote.
As an introduction to post-Roman Britain, "Warlords" is to be recommended above most popular books which concentrate on the doubtful figure of Arthur. However, to be picky, there are a few weak points:
1. Why should we believe that one civitas, Dumnonia, "was quite possibly capable of supplying and equipping twelve thousand men" under the leadership of Riothamus in Gaul, when Leslie Alcock (_Arthur's Britain_) doubts that any of the post-Roman states could have raised even one thousand men.
2. Laycock quotes Roger of Wendover for the relations between Ambrosius and Vortigern even though, as he admits, this is possibly more myth than history. I think this information clearly derives the pseudohistory of Geoffrey of Monmouth.
3. Laycock accuses Bede of "losing the plot" by "allotting Aelle all the territory south of the Humber" but of course Bede ascribes to Aelle only the same authority as he ascribes to the later Ethelbert. There is no reason that a king of Sussex and nearby districts could not have have been held in the same regard as a king of Kent.
So in summary: certainly worth reading, but Laycock's case is not as strong as he makes it out to be.