Question: Please suggest books/references on ARDL model and ARDL bounds test approach to study.

Answer: 
The GRETL progrmme will be more useful, if it includes the test for degeneration case 1 using ARDL Bootstrap method along with PSS and BDM bounds test on cointegration..

McNown et al. (2016) define the following degenerate cases and the degenerate cases cannot be cointegration.
1. Degenerate case #1 occurs when the F-test on joint lagged level variables and the t-test on the lagged independent variable are significant, but the t-test on the lagged level dependent variable is insignificant.

2. Degenerate case #2 occurs when the F-test on joint lagged level variables and the t-test on the lagged dependent variable are significant, but the t-test on the lagged level independent variable is insignificant.

Pesaran et al. (2001) present critical values for case #2, but not for case #1. To rule out degenerate case #1, the integration order for the dependent variable must be I (1).
Btw, there is now a user-written Gretl package for running a bootstrap version of both the PSS and BDM bounds test on cointegration. For an example see [here](https://sites.google.com/site/arturtarassow/it-s-not-really-ablog-but-rather-a-note-to-myself/replicating-eviews-blog-entry-on-ardl-based-cointegration-tests).