Reading is a crucial form of intellectual exchange, but it is also an elusive activity, something that can be done in very different ways. To uncover the history of reading historians look for traces of these different reading practices. Handwritten annotations in books are an important key to understand how historical readers used their books. ABO aims to bring these books together. It is a digital library that reveals the variety of traces that readers left in their books. These examples were previously dispersed over many different libraries in the world. Yet it is also a digital laboratory, where visitors can work together: ABO has tools to enrich the early modern annotations with transcriptions and translations.