To develop a new understanding of how self-directed professional learning (SDPL) contributes to the professional growth of school librarians, a case study design using humanistic learning theory (Maslow, 1954; C. Rogers, 1983) and based on theoretical assumptions about adult learners (Knowles, 1968, 1975, 1980), the intentional self-directed learning of adults (Tough, 1979), and the person, process, context (PPC) model (Hiemstra & Brockett, 2012) was conducted identifying dynamic, intersecting elements of SDPL of school librarians. Specifically explored were required professional development goals, self-directed professional goals, daily professional learning activities, both selected and discarded, and skills or sets of knowledge sought. Data were collected from 20 strong self-directed learning, certified school librarians in a Midwestern state, United States using professional learning diaries covering a two-week period followed by semi-structured interviews. The data revealed the dynamic employment of themes found within the PPC model to facilitate desired, necessary professional learning that far surpassed the professional development structure employed by the school districts that fulfill federal and state laws mandating the professional learning of educators. The findings of this qualitative research inform a new model titled the Learner's Dynamic Self-Directed Professional Learning Guide, which is intended to serve as a supplement to the National School Library Standards (AASL, 2018). This new model was developed to guide school librarians in facilitating their own professional learning as well as to inform administrators, policy makers, and formal educators of essential and flexible elements towards lifelong, professional learning of these specialized educators. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]