Johanna Nichols' "Linguistic Diversity in Space and Time" has become one of the most acclaimed reference tools for the purpose of linguistics. It contains the most accurate and least genetically biased sample that continuing language extinction is ever going to permit us to obtain, and analyses it in an exceptionally effective manner.
Although the number of features analysed is extremely small compared to the vast World Atlas of Language Structures, thye are analysed with considerably more detail and more effort is developed to connect them. For example,
- head marking at the phrase level implies it at the clause level without exception
- the presence of grammatical gender relates to high complexity in terms of number of morphological markers per sentence
- verb-initial order correlates with the presence of abundant head marking on verbs
- stative/active alignment occurs only in head-marking languages
The book also offers a surprisingly detailed analysis of how languages spread and evolve. Nichols shows that in a small number of "spread zones", a single language family dominates, and that this lack of competitiveness causes (or can cause) highly marked features that would not survive in a linguistically more "competitive" environment to flourish. "Residual zones", mainly found on coasts with rich fisheries or in high mountain areas, have high linguistic diversity but tend to confirm to a standard profile of verb-final order and relatively high complexity. In "spread zones", by contrast, other profiles often occur, and the profile of a "spread zone" frequently changes whenever a new family moves through it.
As a bonus, Nichols offers a great many interesting suggestions as to how various regions of the world would have appeared in the distant past. Her knowledge of the ancient languages of the Near East is sufficiently advanced that she can make some highly reasonable but surprising conclusions about, for instance, pre-Indo-European Europe. More interesting still is her suggestion that the recently colonised areas of the Americas and New Guinea represent a much more "primordial" linguistic state than the Old World, where the existence of domesticable animals has increased the intensity of linguistic spreads. Her data, though, do, like WALS, just show how removed the world's major languages are from the typical traits of human language. As an amateur social scientist, I find this remarkably interesting and relevant.
All in all, for those who cannot afford the full book version of WALS, I would wholeheartedly recommed "Linguistic Diversity in Space and Time" though it provides nothing like the same number of features.