Glock's main claim is set out in his "Introduction":
According to the [historical conception], analytic philosophy is first and foremost a historical sequence of individuals and schools that influenced, and engaged in debate with, each other, without sharing any single doctrine, problem, method or style ... [But] a purely historical conception ignores the fact that philosophers can be more or less analytic on grounds other than historical ties. These worries can be laid to rest if we acknowledge that analytic philosophy is a tradition held together not just by relations of influence, but also by overlapping similarities (pp. 19-20).So, Glock offers a hybrid account of what analytic philosophy is. He combines both historical influence with similarities in philosophical commitments.
As I continue to read the book I will be interested to see if Glock addresses a tension that I see in many attempts to characterize analytic philosophy. On the one hand, we want to understand why analytic philosophy developed at the time and place that it did. On the other hand, if we are sympathetic to analytic philosophy, we also want to explain what is good or best about it compared to other developments in philosophy. Both desires can be easily combined if there are conclusive philosophical arguments for certain distinctive views of analytic philosophy, and these arguments were presented by the early analytic philosophers. But, in my experience at least, these things are very hard to find. As a result, many historians feel forced to choose either a purely causal reconstruction of historical developments or else a timeless reconstruction of philosophical arguments. Some third alternative is clearly needed.