You pose a very broad question here are a few suggestions. There is a lot of exciting work being done on happiness. I'd suggest starting with the work of Dr. Ruut Veenhoven, a pioneer in this field. Peter Warr's book The Psychology of Happiness and Andrew Steptoe's work on Happiness and Health also come to mind. You might also browse recent issues of Journal of Happiness Studies. An Interdisciplinary Forum on Subjective Well-Being. Good luck with your work!
There are two recent articles discussing the future of happiness research in economics. Each is written by one of the most prominent scholars in the field.
Andrew E. Clark (2018) "FOUR DECADES OF THE ECONOMICS OF HAPPINESS: WHERE NEXT?" Review of Income and Wealth, doi.org/10.1111/roiw.12369
Bruno S. Frey (2019) "What are the opportunities for future happiness research?" International Review of Economics, doi.org/10.1007/s12232-019-00318-9