Yes, Perry Mason and Della Street were romantically involved in the books and implicitly in the TV show. In the books, Perry Mason even proposed to Della on several occasions, but she turned him down because she wanted to continue being his secretary, and he believed having his wife as his secretary would look bad [1].

In the TV show, their relationship was portrayed as deeper than that of an employer and his secretary. While their cases took center stage, there was always an underlying romantic tension between them. Perry Mason proposed to Della multiple times, but she consistently turned him down, understanding that he wasn't the type to settle down, and she didn't want to spend her life without him as a housewife and mother [2].

Their on-screen chemistry and the intuitive acting of Barbara Hale, who played Della Street, contributed to the seething romance between the characters. The relationship between Perry Mason and Della Street continued to captivate fans until a kiss in the 1993 TV movie "The Case of the Telltale Talk Show Host" finally confirmed their romantic connection [2].

Learn more:

1. Were Perry Mason and Della Street an item? - Quora
2. Della Street and Perry Mason romance | Talking Classics
3. Inside the ‘Perry Mason’ Della and Anita Lesbian Love Story