I'm looking to learn about the history of semicha. I've read a few articles online, but I'm really looking for something more in depth - a book preferably, or at least a long journal article. Must be English. Any suggestions?
Before you mentioned the English requirement, I would have suggested the articles published here, and since they're good articles, I'm going to keep them as part of the answer anyway. Additionally, in Hebrew, there is the great book by Yisrael Yuval, חכמים בדורם... but since you're looking for books in English, I'm afraid the only one that I know of is "Semikhah (Ordination): A Study of Its Origin, History, and Function in Rabbinic Literature" by Julius Newman. Don't be disappointed though- he covers a period wide enough that there's very little in depth analysis or discussion, and the book is meant as more of an introduction (I think), nor does he really discuss the question of semikha vs. minuy shoftim or the like.
Even though you said that you've seen articles on the topic, the best is probably Lawrence Hoffman's article, "The Origin of Ordination" which was published in "Rabbinic Authority: Papers Presented Before the Ninety-First Annual Convention of the Central Conference of American Rabbis".
If you'd really like to do an in-depth study, and you still don't want anything in Hebrew, then the next best option is to look at articles that discuss particular changes, innovations and developments that occurred throughout history regarding semicha, such as the forms of semikha mentioned in the beginning of Maseches Sanhedrin, how the Geonim viewed their 'heter horaa', the controversy surrounding R. Yaakov Berav, and the Rabbinate as viewed by their own members in every time and place from 12th century North Africa to 20th century England and everywhere in between. What the word 'semikha' even means has varied over the many times and places, with the most major difference noted in the comments (that today 'semikha' refers to a heter horaa but that there's a whole separate form of semicha required for certain halakhic rituals and decisions)