1Mobilité sans racines was written by a group of authors (Vincent Kaufmann, Stéphanie Vincent-Geslin, Gil Viry, Nathalie Ortar and Iragaël Joly) interested in the development of new forms of mobility in Europe, specifically, long-distance commuting and job-related dual residence. Mobility, here defined as “moving ever-greater distances at ever-increasing speed,” is now valued as a skill for overcoming constraints related to the increasing distance between and dispersion of places of activity. With regard to work commutes, my primary focus here, the spatial practices observed raise questions of rootedness and people’s attitudes toward it, but also of changes in social and family relations due to individuals’ temporary absence from their main place of residence or family home.
2According to the authors, what characterizes European worker mobility now is that people “use territories and transport systems to go back and forth – that is, they use them reversibly.” The contributing authors share an approach based on the concept of “reversible mobility.” Findings from the studies they cite indicate that this type of mobility is widespread in Europe: half of the continent’s population practices it if past as well as current individual situations are taken into account. The specificity of reverse mobility is not necessarily related to changes in work but rather to the new transportation networks, which help increase the speed of getting from place to place and may even seem to abolish distance altogether. The practice has given rise to a relatively new social category: individuals with substantial commute-time budgets that they no longer necessarily seek to diminish. It also involves optimal use of telecommunications networks, which in turn impacts on people’s attitudes toward space and on their social relations.
3Lastly, reversible mobility meets a growing need for reconciling work and private life at a time when the different activities they involve are becoming increasingly separated in space. Work is not the only thing taken into account in the decision to become a “long-distance commuter”; what seems to count most is individuals’ attachment to the territory they reside in and their social life there. Reversible mobility therefore actually enables people to remain sedentary. Achieving work-life balance is not only a matter of working couples’ making geographic compromises, nor is reversibility necessarily viewed as a constraint; it may very well be a chosen practice. In such cases, it “palliates uncertainty and constitutes a bulwark against flexibility and change.”
4The analyses presented in the work’s six chapters draw on datasets from a variety of qualitative and quantitative surveys that developed out of several research programmes; the survey of long-distance commuters in Germany, Belgium, Spain, France, Poland and Switzerland (part of the European Job Mobility and Family Lives programme) is the one most cited in the argumentation here. While clearly the book’s brevity does not allow for a long methodological demonstration, the authors might have provided more information on the sampling techniques used, respondent profiles and geographic sector characteristics.
5If, as can reasonably be assumed, respondents were primarily individuals living in or with attachments to an urban, if not metropolitan, trans-border area at the “economic heart of Europe,” then that context specificity may have significantly skewed the results. To what degree can these findings be generalized and used to demonstrate the emergence of a new lifestyle or social category in Europe? On this particular point, it would be useful to compare the authors’ findings with other recent ones on the same topic from studies of different mobility contexts that use different methodologies. 
6Though in their preliminary account the authors seem to regret the “gradual fragmentation of mobility studies within the social sciences” (p. 20) and though they call for an “integrative approach to mobility” (p. 24), their own approach is firmly sociological and their perspective Eurocentric. When it comes to the questions of abolishing distance with speed or the new territorial characteristics, they fail to take into account existing geographic studies on those subjects,  or demographic studies and findings based on a longitudinal, multidimensional approach to mobility. 
7The general bibliography lacks certain references that would support the authors’ arguments for opening up mobility studies. For over 20 years in fields in both Northern and Southern countries (especially West Africa, India, Latin America), French-speaking geographers, demographers and sociologists have been developing a comprehensive, integrated approach to different types of spatial mobility, an approach that does not use any pre-existing typology. The “hybrid” and/or “reversible” types of mobility discussed in Mobilité sans racines have already been studied, though not in isolation from other types of movement. On reading this text, then, the reader wonders how the types of movement observed in it (long-distance commuting, employment-related dual residence) fit into larger systems of individual and family mobility. In other words, how do those practices emerge and change in relation to the other ways and reasons individuals move about? And how are not only couple relations and the places involved in job-related movements or commutes but also social relations more generally and relations within the family circle organized?
8In addition to the research avenues the authors indicate for “pushing forward” our understanding of mobility phenomena, it would perhaps be useful to examine how the micro (individuals, families), meso (neighbourhoods, towns, communities) and macro levels (the state, regions) operate together, with special attention to context effects. 
Cf. Christophe Imbert et al., D’une métropole à l’autre: Pratiques urbaines et circulations dans l’espace européen [From one metropolis to another: urban practices and circulation in European space], Paris, Armand Colin, Recherches, 2014, 485 p.
See, for example, Michel Lussault, L’homme spatial: La construction sociale de l’espace humain [Spatial man: the social construction of human space], Paris, Seuil, La couleur des idées, 2007, 364 p.
See the Groupe de Réflexion sur l’Approche Biographique [GRAB; study group on the life course approach], Etats flous et trajectoires complexes, observation, interprétation, modélisation [Fuzzy states and complex trajectories: observation, interpretation, modeling], Paris, INED, Méthodes et Savoirs no. 5, 2006, 301 p.
Cf André Quesnel, “De la communauté territoriale à l’organisation familiale en archipel” [From the territorial community to family organization in the form of archipelagos], in Françoise Dureau and Marie-Antoinette Hily, eds., Les mondes de la mobilité [Mobility worlds], Rennes, Presses Universitaires de Rennes, 2009, pp. 68-103.