Open access infrastructure for books is becoming more mature, and it is being used by an increasing number of people. The growing importance of open access infrastructure leads to more interest in sustainability, governance and impact assessment. For this paper, we will assess the OAPEN Library. In the spring of 2022, it passed the milestone of 20,000 titles. This was a good moment to evaluate the core asset of the OAPEN Library: its collection. How well meets the collection the needs of its users? The OAPEN Library sees global usage; the collection reflects this by offering titles in over 50 languages. The collection is not focused on a specific subject area, but the choice of medium – books and chapters, not journals and articles – is more strongly associated with the humanities and social sciences. It does not track its users, but the supporters of the OAPEN Libraries are globally distributed academic institutions, scientific and scholarly funders and publishers. An assessment of the OAPEN Library should therefore take into account the diversity of languages, subjects and users.