Though not open access, a digital version of the Merck Manual Professional Edition is freely available from the publisher.
Practical Clinical Skills is a website providing free training and reference guides, including simulation-based lessons on heart sounds, murmurs, lung sounds, carotid bruit, blood pressure measurement, and EKG interpretation. Free registration is offered but not required to access most materials.
Uchmanowicz, I., Jankowska-Polańska, B., Wleklik, M., Lisiak, M., & Gobbens, R. (2018). Frailty syndrome: Nursing interventions. SAGE Open Nursing 4. https://doi.org/10.1177/2377960818759449