The Adelphi book series, the Institute's principal contribution to policy-relevant, original academic research, has adopted a strikingly new style. Adelphis are now short books, rather than long monographs; their average length will be 40,000 words (compared with the 28–30,000 of the previous series). The new style also involves a modernised appearance, including individually designed cover art for each new book. Japan’s Remilitarisation by Christopher W. Hughes and Sarah Raine's China's African Challenges are the the first books in the new-look series.

 As before, approximately eight books are published in the series each year, designed to provide rigorous analysis of strategic and defence topics that will prove useful to academics, researchers, politicians and diplomats, as well as foreign-affairs analysts, defence commentators and journalists.

Ever since the publication of Alastair Buchan’s The Evolution of NATO in 1961, the Adelphi series has been the principal contribution of the IISS to policy-relevant, original research on strategic studies and international political concerns. Over the years, Adelphis have provided high-quality analysis of key security issues, serving to inform opinion, to stimulate debate and to challenge conventional thinking. The series permits the IISS both to remain responsive to events and to contribute significantly not only to debate on strategic affairs but also to the development of policy.