The work Ezras Kohanim by R' Yehoshua Yosef HaKohen (originally printed in Warsaw, 1873) is an indispensable tool and resource for any serious student of Tractate Middos. The author cites an impressive variety of sources related to the Mishnah, many of them hard to find and/or out of print, and spends much time analyzing the various opinions, pointing out the strengths and weaknesses of each. There are also numerous diagrams throughout the text.
This work was recently reprinted in a two-volume set by Mifal Torah Hamikdash, under the guidance of Rabbi Avrohom Yeshayah Neuwirth. The difference between the printings available up till now and this new one is like night and day and much credit is due Rabbi Neuwirth for all of his efforts in making this remarkable sefer more accessible to the public.