To cite ideas from this collection of materials, you can either cite the book in general, or the online materials specifically. Here are examples using APA format.
The book
Borgatti, S. P., Everett, M. G., Johnson, J. C., & Agneessens, F. (2022). Analyzing Social Networks Using R. SAGE Publications Ltd.
The online digital resources
Borgatti, S. P., Everett, M. G., Johnson, J. C., & Agneessens, F. (2022). Analyzing Social Networks Using R: Online Digital Resources. SAGE Publications Ltd.