Hi Can you tell me where the custom of saying Baruch Hu Ubaruch Shemo originated from? Who started it? Is it mentioned in Talmud? In which Sefer can I find some insight into this?

Shalom, Thank you for your question. The practice of responding "Baruch Hu U'Baruch Shemo" is found in the writings of the Rosh (who lived in the 1200's). He writes that his father would say these words (Blessed be Him and Blessed be His name) after hearing the mention of G-d's name in a blessing. (See Rosh, responsa, 4, 19). This practice is based on the words of the Torah (Devarim 32,3) "When I call out the name of the L-rd, ascribe greatness to our G-d. " After hearing someone "call out the name of the L-rd" we then "ascribe greatness" to that name of G-d by saying the words Baruch Hu etc. This practice is recorded as standard law in the Shulchan Aruch (Orech Haim, 124, 5). I am so glad to see that you asked for a sefer to learn more about this idea. Whilst we enjoy providing answers to questions on this site – our greatest joy is helping people go on to learn themselves. A good place to begin your studies about this would be to open up the Shulchan Aruch with the Mishna Brurah and learn the sources. You could trace it's sources back up through the Tur and Beit Yosef (with the same reference source in Orach Haim). From there I am sure you will be able to find other reference material. (If your own learning level isn't up to this with the original hebrew texts – this is a great opportunity to approach your Rabbi (or other learned person) and set up a time to learn together). May you have every success in your studies, and may the mitzvah of your learning Torah bring much light to the world. Blessings.