Deadly Nightshade is poisonous but was apparently used for a while by Venetian women as a cosmetic product to redden their skin and dilate their pupils, a look that was fashionable at the time, which is how it got the name Atropa Belladonna (bella donna meaning beautiful lady). In addition to its physical symptoms, the Deadly Nightshade's symptoms include hallucinations and delirium.
Here are my questions:
Could it have been used in the 1800s (when the novel is set) in small doses over an extended period of time to give the impression that someone is going mad?
And also, if someone stopped taking it, would they recover quickly?
Otherwise, what other substance known to man in the 1800s would have similar effects on someone's mental state?
I've been doing a lot of research online, but can't find very specific information about the process of going through this kind of poisoning. If anyone has some knowledge on poisonous plants and has some info for me, I'll be so thankful!
Though there is an historical element to your question to which someone might be able to respond, you might ask on one of the science oriented reddits as well (r/askscience or r/botany perhaps) for information on the biological effects of nightshade.
Belladonna is an anticholinergic; its toxicity is well-described. The major symptoms would be a very dry mouth, a racing heartbeat, urinary retention, and delirium, especially waxing and waiting mental status. Withdrawal of the substance wood lead to recovery.
The other drugs in the material medica of the late 19th century that might fit the bill would be digitalis (foxglove), which was commonly used and has a notoriously narrow therapeutic index. It can cause hallucinations and xanthopsia (yellow vision). We still use it (as digoxin) today. Then there's gelsemium, which acts overall as a depressant.
Honestly, the drugs you're really looking for are arsenic (or any of the aniline dyes that were all the rage) and to a lesser extent mercury (calomel, or so called sweet mercury). The neuropsychiatric effects of both of these drugs can cause paranoia, hallucinations, agitation, &c. Think the main character from S*town. Unfortunately there would not be rapid recovery with either of these (or really much of a recovery at all at this stage).
Edit: not sure what period you're looking for, but if in the US a semifictional tonic or patent medicine would be a good idea. Im not personally that knowledgeable in this arena, but many contained potentially psychotropic substances (though mostly opium and alcohol). So you could have Dr Meisters Cure All, which contained canabis, opium, alcohol, and digitalis -- which could presumably achieve symptoms like you're going for.