Chaoluan Kao's Reformation of prayerbooks studies Protestant prayerbooks in England and Germany from the mid-sixteenth to the early eighteenth century. Chapters i and ii trace the emergence of a distinctively Protestant prayerbook from medieval Catholic models, while chapters iii, iv and v explore how Protestant authors, influenced by Renaissance humanism, turned away from the medieval emphasis upon contemplation to emphasise the importance of sola fide and worldly vocation. Chapter vi examines the role of prayerbooks within women's piety, and chapter vii details how godly English prayerbooks influenced the development of German Pietism. Reformation of Prayerbooks concludes with multiple tables, including one (Table 4) that will be of considerable use to subsequent scholars, since it lists forms of prayer and whether they are included in at least twenty-six German and English prayerbooks.

Kao is at her best when detailing points of convergence between English and German prayerbooks. Chapter vii fruitfully places works like Lewis Bayly's Practice of piety (1613) and Joseph Hall's Arte of divine meditation (1606) in a transnational perspective and underscores the shared values of German and English piety. Kao also displays a keen awareness of the importance of prayerbooks to the spiritual life of the believer. She demonstrates how reading prayerbooks was considered a form of prayer, how prayerbooks were used by women to strengthen their relationship with God, and how the use of prayerbooks provided the believer with a way of structuring his or her daily life. This focus upon the importance of prayer to the spiritual life of the believer leads Kao to make some illuminating and unexpected connections, demonstrating points of convergence between such apparently divergent works as Gerhard's Meditationes sacrae (1603) and Taylor's Holy living (1650). Similarly, she observes that Martin Luther and Francois de Sales ‘shared similar notions about vocation’ (p. 126). Scholars are increasingly recognising that because prayerbooks are less concerned with doctrine than devotion, they often cross confessional boundaries; Kao's analysis brings this point home.

One wishes, at times, that more attention had been paid to the particularities of individual prayerbooks. For example, chapter vi convincingly underscores the centrality of prayerbooks to the spiritual life of early modern women and their importance in the education of children. But Kao's choice of prayerbooks is quixotic. When discussing English women as authors of prayerbooks, she analyses the Catholic writer Elizabeth Grymeston at length, but she does not discuss in detail the arguably more influential writings on prayer by Elizabeth Jocelin or Dorothy Leigh. More significantly, Kao only discusses printed prayerbooks, not manuscript prayers. This may be a deliberate choice, but it ignores the fact that many of the women's prayerbooks that she discusses (for example, those of Elizabeth Grymeston and Elizabeth Richardson, countess Morton) were first composed in manuscript and only subsequently printed, arguably as forms of political intervention. Acknowledgement of the complex relationship between print and manuscript prayerbooks would have strengthened the book. Similarly, Kao's analysis does not always rely upon the best or even the most recent scholarship; for example, when discussing Katherine Parr's Lamentation of a sinner (1547) she does not cite from Janel Mueller's magisterial edition of Parr's work (2011) which explores the relationship between print and manuscript in detail. (Kao does include Mueller's edition in her bibliography.) Similarly, she fails to cite the voluminous scholarship of Micheline White on Parr's prayers. Finally, Reformation of prayerbooks is marred by repeated grammatical errors and typos, although none is so serious as to prevent comprehension.

Kao's Reformation of prayerbooks is more interested in continuity and consensus than in originality in styles of prayer. While the book downplays the particularities of individual prayerbooks, Kao's transnational and chronological breadth is admirable and her focus upon the importance of prayer for the spiritual life of a believer helps to explain the popularity and longevity of these works.