Here's a popular one:

Hayes, A. F. (2013). Introduction to mediation, moderation, and conditional process analysis: A regression-based approach. Guilford Press.

However, web pages and youtube videos might provide a better introduction if you find that this book is a bit overwhelming.

I agree that you could start with resources on the web. David Kenny is one of the originators of the terms mediator and moderator, and he has an excellent website:

http://davidakenny.net

Hello,

Please follow the link attached

Regards

https://books.google.ie/books?hl=pl&lr=&id=iWFSpQFh-y4C&oi=fnd&pg=PP1&dq=moderation+and+mediation+book&ots=1y15sHYIPX&sig=DYYr-xI6WCaexKOhB42_1CqiW5g&redir_esc=y#v=onepage&q=moderation%20and%20mediation%20book&f=false

This might be of interest to you - as a gentle introduction to the concepts and to literature in the field: https://www.researchgate.net/publication/299598926_Mediation_Moderation_and_Interaction_Definitions_Discrimination_and_some_means_of_testing

Presentation Mediation, Moderation, and Interaction: Definitions, Discrim...

I am thankful to all of you for suggestions and material.