A quick go-to resource I have found more than helpful has been a series that was published on Microsoft's technet blogs. They've done a great job presenting search from the bottom up. From creating result sources to creating verticals to improve your client's search experience. Also, from playing around with display templates to custom actions on the preview panel.
The series has many topics and subtopics and can become a bit annoying if you're trying to navigate to get to a specific topic in the series. Because of this, I created a table with all the main topics and subtopics included in each post. I hope this is helpful and time saving if you decide to use this as a reference.
May this help you find your way sooner than later