Is there a volume in English that explicates the relationship between each haftarah and its parsha?
Failing that, is there series of commentaries on the portions that routinely includes discussion of the haftarah?
According to the publisher:
In this masterpiece, the Rosh Yeshivah introduces each Haftarah, explains its historical context where necessary, shows its relationship to the Parashah, and offers an enlightening commentary in his own unique, original manner.
I think the Hertz Chumash does a good job.
Keep in mind, though, that not every Haftarah has a direct relationship to the Torah parsha or even a special occasion occurring on that day. For example, the 7 Haftarat of "Consolation" that occur between the week after Tish'a B'Av (Shabbat Nachamu) and prior to Rosh Hashanna (Netzavim or Netzavim / Vayelech) are mot directly related to the parsha reading.
My favorite is the commentary of R' Samson Raphael Hirsch on the haphtaros, he often connects the two.
In addition, the classic Stone Chumash from ArtScroll has a note on every haphtorah, often explaining the non-obvious connections.
The Samson Raphael Hirsch chumash set has a volume dedicated to the haftorah, many times explaining the connecting theme.
In Hebrew, there is Rabbi Shimon Schwab's commentary on the chmash Maayan Beis Hasho'eva which will at times focus on the haftorah independently.
Meshech Chochma, also Hebrew, will at times have insights into the haftorah as well.