I agree that "slogging hard work" looks strange. I have not seen "slogging" used as an adjective. Even if it were possible, it would be redundant as it would add nothing to "hard". As you say, "slogging" alone is enough.

I am grateful for your answers. I think that the novel is written in American English. I imagine that "slog" probably mean unpleasant in the novel I am reading. Thank you.

I checked Merriam Webster and "slogging" is not an adjective there. The definition of "slog" (noun and verb) is the same in the UK and the USA, according to the dictionaries, and also my experience. The writer in this case has coined a new adjective "slogging", and perhaps it will catch on one day. In the meantime, it's definitely non-standard with me, and also non-standard according to standard US and UK dictionaries.

It is not redundant because they have different meanings. Anyway in all forms of non technical writing it is common to use more than one adjective, even if they are close in meaning, to show emphasis ( ie, the bright shining sun)

I guess Suki is right, people are very keen to "resolve" questions without waiting for other answers. "slog " is fairly common where I live. slogging refers to a duration of time , hard applies to a level of difficulty and intensity; slogging may be unpleasant, but not necessarily difficult: "It will take years to slog through this mess of disorganized documents." "It's hard to lift 1000 kg." "Its going to be a long slog through this snow to get home."