Supporting References for my analysis choice level vs. difference regression


I need some help in finding some good references which help me to document the regression approach I used.

I am analyzing the influence of corporate credit ratings (AAA, AAA-,AA+,AA,AA-, etc.)on corporate strategy. Corporate strategy is measured by the diversification level of the firm. In this respect I have performed to analyses:

(1) Investigation of the relationship between credit ratings and corporate strategy. Here, the idea is to check whether the diversification level increases with a higher credit rating. Therefore, I use level regressions and regress the level of diversification on the level of the credit rating and some other control variables.

(2) When a credit rating is in a positive (A+) or negative (A-) rating state, the probability to get an upgrade or to suffer a downgrade is higher compared to a stable rating Situation (A). Consequently, the idea is that firms are more likely to change the diversification level in these rating situations compared to a stable rating Situation. To measure this effect, I used difference regression and check whether there is a positive or negative change in the diversification level in case the firm is in a positive or negative rating situation. The control variables applied are also used in differences.​

I would like to document the choosen statistical approach with some references. Therefore, I really would appreciate if you have some recommendations on references which analysed a similar relationship (maybe not credit ratings but similar in its basic idea) and share the reference titles with me so that I can document it in my paper.

Thank you very much in advance and please let me know in case of questions or comments.