what's the meaning of a crack brain scribbling bookseller?

It would help if you gave a bit more context, if we knew who 'him' was, it would help. Crack brained normally means stupid or unhinged. Scribbling would indicate a writer of some sort.

Google reveals...
The"him" was John Dunton and D'Israeli was the father of the PM of that name. Both were authors, with the latter probably envious of the relative success of the former.

I assumed D'Israeli was the pm, not his father, which explains why it's always a good job to give more context. I think you're quite right about the envy, but I've never heard of John Dunton, although I do know that D'Israeli was a writer as well as pm.