I have just finished the book "Tom M. Apostol - Introduction to Analytic Number Theory". My aim is to reach to graduate level to do research, especially on Rationality/Irrationality and Algebraic/Transcendentality of Euler-Mascheroni Constant, but articles are not only too advanced to study after Apostol's book, but also I don't think that they are readable by just studying Apostol's book at all for a self-learner like me. Please someone tell me a book for Analytic Number Theory more advanced than Apostol's book but readable esp. for self-learning . Thanks a lot.
PS I am not sure if the books recommended here could be more helpful for research on Euler-Mascheroni Constant. I have also studied the book Havil, J.: Gamma: Exploring Euler's Constant and some elementary materials in continued fractions..