Can anyone recommend a good sefer explaining Klalei Horo'oh - how a Rov reaches a psak, being machria between different acharonim etc?
The Sefer Yad Malachi is an extremely important work ,and the Shach's(Yoreh Deah 242) klalim on psak. There is also an amazing Kuntres that can be found in the back of volume 1 of Sheaarim Metzuyanim B'halacha on shas that discusses seder toras halimmud, and toras horaah and minhagim. Alot of great mareh mekomos and interesting opinions on the subject(see number 8 about the Birkei Yosef's opinion of paskining from seforim). For Chacham Ovadia's understanding of derech psak see the beginning of the first volume .