This book constitutes the refereed proceedings of the 21st International Conference on Asia-Pacific Digital Libraries, ICADL 2019, held in Kuala Lumpur, Malaysia, in November 2019.The 13 full, 13 short, and 5 poster papers presented in this volume were carefully reviewed and selected from 54 submissions.

Text Classification.- PCGAN-CHAR: Progressively Trained Classifier Generative Adversarial Networks for Classification of Noisy Handwritten Bangla Characters.- Weakly-Supervised Neural Categorization of Wikipedia articles.- Supervised Classifiers to Identify Hate Speech on English and Spanish Tweets.- Altmetrics.- An Overview of Altmetrics Research: A Typology Approach.- User Motivation Classification and Comparison of Tweets Mentioning Research Articles in the fields of Medicine, Chemistry and Environmental Science.- Understanding the Twitter Usage of Science Citation Index (SCI) Journals.- Scholarly Data Analysis and Recommendation.- Research Paper Recommender System with Serendipity using Tweets vs. Diversification.- The rise and rise of interdisciplinary research: Understanding the interaction dynamics of three major fields – Physics, Mathematics & Computer Science.- The Evolving Ecosystem of Predatory Journals: A Case Study in Indian Perspective.- Metadata and Entities.- Identifying and Linking Entities of Multimedia Franchise on Manga, Anime and Video Game from Wikipedia.- Impact of OCR Quality on Named Entity Linking.- Authoring Formats and Their Extensibility for Application Profiles.- Digital Libraries and Digital Archives Management.- Connecting Data for Digital Libraries: The Library, the Dictionary and the Corpus.- Modernisation of a 12 years old Digital Archive: Experiences and Lessons Learned.- Investigating the feasibility of digital repositories in private clouds.- Multimedia Processing.- Towards Automatic Cataloging of Image and Textual Collections with Wikipedia.- Automatic Semantic Segmentation and Annotation of MOOC Lecture Videos.- Play, Pause, and Skip: touch screen media player interaction on the move.- Search Engines.- Towards A Story Based Search Engine for Arabic Speaking Children.- Metadata-based Automatic Query Suggestion in Digital Library using Pattern Mining.- Investigating the effectiveness of client-side search/browse without a network connection.-  SchenQL: A Concept of a Domain-Specific Query Language on Bibliographic Metadata.- Information Extraction.- Formalising Document Structure and Automatically Recognising Document Elements: A Case Study on Automobile Repair Manuals.- Weakly-Supervised Relation Extraction in Legal Knowledge Bases.- Social Media Aware Virtual Editions for the Book of Disquiet.- Language identification for South African Bantu languages Using Rank Order Statistics.- Posters.- Creating a Security Methods Taxonomy for Information Resource Management.- A Dynamic Microtask Approach to Collecting and Organizing Citizens' Opinions.- Sentiment Analysis of Tweets Mentioning Research Articles in Medicine and Psychiatry Disciplines.- Measurement and Visualization of IIIF Image Usages.- Improving OCR for Historical Documents by Modeling Image Distortion.