Here is the second part of the list, see here for the first part. These references focus mostly on methods of trend estimation, smoothing and filtering and not so much on the concept of the trend itself. The list covers a medley of estimation and inference approaches and should be sufficient for looking into additional references on the topic. Enjoy!

1. Dynamics of Exponential Smoothing with Trend and Seasonal Terms, Management Science, 1974.
2. Sequential Signals of Recession and Recovery, The Journal of Business, 1982.
3. The Prediction of Time Series with Trends and Seasonalities, Journal of Business & Economic Statistics, 1983.
4. Unobserved-Components Models for Seasonal Adjustment Filters, Journal of Business & Economic Statistics, 1984.
5. Forecasting Trends in Time Series, Management Science, 1985.
6. On Structural Time Series Models and the Characterization of Components, Journal of Business & Economic Statistics, 1985.
7. A New Approach to the Economic Analysis of Nonstationary Time Series and the Business Cycle, Econometrica, 1989.
8. Stochastic linear trends: Models and estimators, Journal of Econometrics, 1993.
9. Estimating Deterministic Trends in the Presence of Serially Correlated Errors, The Review of Economics and Statistics, 1997.
10. Trend estimation and de-trending via rational square-wave filters, Journal of Econometrics, 2000.
11. Signal extraction and the formulation of unobserved components models, The Econometrics Journal, 2000.
12. Methodology for Trend Estimation, Economic Modelling, 2001.
13. An Unobserved-Component Model with Switching Permanent and Transitory Innovations, Journal of Business & Economic Statistics, 2005.
14. Challenges in Time Series Econometrics, Mathematics and Computers in Simulations, 2005.
15. Econometric methods of signal extraction, Computational Statistics & Data Analysis, 2006.
16. Time series decomposition and measurement of business cycles, trends and growth cycles, Journal of Monetary Economics, 2006.
17. Estimating Trends with Percentage of Smoothness Chosen by the User, International Statistical Review, 2008.
18. Inference on Nonparametrically Trending Time Series with Fractional Errors, Econometric Theory, 2009.
19. The Mysteries of Trend, Cowles Foundation Papers, 2010.
20. Signal Extraction Revision Variances as a Goodness-of-Fit Measure, Journal of Time Series Econometrics, 2010.
21. Trend filtering via empirical mode decompositions, Computational Statistics & Data Analysis, 2013.
22. Signal Extraction for Non-Stationary Multivariate Time Series with Illustrations for Trend Inflation, Journal of Time Series Analysis, 2014.
23. Nonparametric estimation of a periodic sequence in the presence of a smooth trend, Biometrika, 2014.
24. Core Inflation and Trend Inflation, The Review of Economics and Statistics, 2016.
25. Steady state adjusting trends using a data-driven local polynomial regression, Economic Modelling, 2019.
26. The trilemma between accuracy, timeliness and smoothness in real-time signal extraction, International Journal of Forecasting, 2019.
27. The Multivariate Linear Prediction Problem: Model-Based and Direct Filtering Solutions, Econometrics and Statistics, 2020.