I am a pharma BSc so the biostatistics textbooks I already read are probably second nature to you. Anyway textbooks that I have ~~not read yet~~ found to be geared toward ~~above-my-IQ-level~~ advanced clinical trial statistics are:
Clinical Trial Optimization Using R seems to focus on design
Clinical trials in oncology which emphasizes applications in oncology but many of the described methods are used elsewhere
Clinical trials: study design, endpoints and biomarkers, drug safety, and FDA and ICH guidelines. The most comprehensive of them all. You will probably be familiar with most of the statistical methods describes here. It is more oriented toward medical literature, describing (and explaining) Endpoints, and guidelines. depending on your needs it might be the most or least convenient textbook. To a person with a medical (and not mathematical) background like me, that would be the most legible textbook on this list.
Sample Size Calculations in Clinical Research. Sample size calculation for literally every imaginable scenario.
Oxford Handbook of Medical Statistics. I actually read this one. I think that would be a practitioner's quick refresher or catalogue of statistical methods. It does not delve deep into any specific topic. I guess you don't want that but I included it for the sake of completeness.
But again don't take my recommendations for granted because I haven't read all of them entirely. They are just the textbooks I would go to if I were in your position with my background.