Are there novels originally written in Esperanto that are considered relevant in literature in general (for their own merit, and not only as "curiosities" or playful experiments which would be considered poor had they originally been written in a natural language)? I would be very interested in finding something like this: Works whose value is deemed high in academics studies as pieces of literature, and not just interesting because they are written in Esperanto. I'd like to get some Esperanto novels for next holidays. :)
This is probably difficult to answer neutrally.
On one hand, I would assume that most serious Esperantologists (linguists exploring Esperanto) are not just Esperantists (Esperanto speakers) but also arguably "samideanoj" (proponents of Esperanto and / or its "interna ideo"). Their judgment has therefore to be assumed to be biased.
On the other hand, I'm not aware of any rating or serious discussion of originally Esperanto works of fiction by literature experts who aren't themselves Esperantists. Due to lack of visibility of Esperanto literature in the wider literary community, it could well be that only few or even none such ratings or discussions exist. (If anyone is aware of any, I'd be interested in them, too.)
This is off course not a problem specific to Esperanto literature alone. It has for example been criticized that the Nobel Committee for Literature at the Swedish Academy is staffed mostly or completely with Europeans and that it thus inadequately considers high-quality literature in languages or from cultures that westerners don't tend to understand, know or be familiar with.
Follow the money
If there were works originally written in Esperanto, of which translations into other languages are popular or even economically successful, that could be an indication that those works are indeed good, even in international comparison. I'm not aware of any, so I've posted a separate question: Are there any originally Esperanto works of fiction with a significant non-Esperantist readership? (Off course, the actual or apparent lack of any such works doesn't have to imply that all Esperanto literature is bad in international comparison. It could again be that it lacks deserved visibility outside of Esperantujo.)
Recommendations by fellow Esperantists
While their judgment on the literary quality might be biased, if you just want to find some worthy reads in Esperanto, it's probably a good idea to consider the opinion of fellow Esperantists.
Marcos Cramer's answer to Is there an Esperanto literary canon? points to a list of recommended literature compiled by the Scottish Esperanto author William Auld. While some of it is poetry (it seems Auld himself was a poet), it contains some novels, too.