RF circuits
It depends if you want to go for RFIC field, first you should learn analog IC design and after the RF specified circuits. If you don't have solid understand in analog design you will feel frightened. There are many books in this topic. What is the best for you it depends on your pre-studies. n And believe me nowadays, for an RF designer needs good understand in many fields like analog design, DSP... this field is developing very fast, there are always new challanges.