This is the best way I can think of at the moment to express what I believe it means. Perhaps someone will come up with a better idea.

"Por contra" is an established, though not very common, expression in European (not American) Spanish for "in/by contrast" or "on the other hand". It is almost certainly a calque of the French par contre. So it's a synonym of "en cambio" or "contrariamente". Fundéu considers it acceptable, but recommends using an alternative:

"Por el contrario, en cambio o contrariamente son alternativas en español a por contra, locución también aceptable en el español de España.
En los medios de comunicación es habitual encontrar frases como «El Real Madrid ganó en su campo el pasado sábado; por contra, el Atlético perdió» [...]
Pese a que tradicionalmente se ha venido censurando el uso de por contra por considerarse un calco del francés, se trata de una locución plenamente asentada en España y aparece recogida en el Diccionario del español actual, de Seco, Andrés y Ramos. Su uso resulta extraño en América, sin embargo, donde se prefieren las alternativas mencionadas."
https://www.fundeu.es/recomendacion/por-contraen-espanol-se-...

"Por contra de", by extension, basically means "in contrast to" or "contrary to". It is unusual (and indeed has no equivalent in French, where par contre de is not used in this way). But occasional examples can be found:

"Por contra de lo que ocurrió en Andalucía tras sus elecciones autonómicas, despertando la movilización de numerosos colectivos al conocer los resultados, en Valladolid han querido salir a la calle antes de las generales del 28A"
https://kaosenlared.net/video-y-fotos-el-movimiento-antifasc...

"El activista alemán ha declarado que, por contra de lo que el Tribunal ha alegado, no es un crimen sostener una fotografía de Öcalan"
https://anfespanol.com/europa/activista-aleman-seguire-exigi...

"la mayoría de delegados norteamericanos preferían la propuesta liberal, por contra de aquellos que provenían de África, Europa y Asia."
https://www.evangelicodigital.com/mundo/6025/iglesia-metodis...

So the basic idea here is the contrast between the widespread circulation of fiction in print and the existence of collections that were not printed. The "colecciones de textos no impresas" bucked the trend, as you might say. I think you might express this as "running counter to"; I'm less sure that "in contrast to" or "contrary to" really work in this context, but you might consider these too.

The other point is that, as is often the case, "relevance" is not the right word for "relevancia" here. The basic meaning of "relevancia" is importance or significance:

"relevancia
1. f. Cualidad o condición de relevante, importancia, significación."
https://dle.rae.es/?id=VpxMLGU

"relevante
1. adj. Sobresaliente, destacado.
2. adj. Importante, significativo"
https://dle.rae.es/?id=VpzDZF6

Relevant means pertinent or germane: related to the matter at hand. Sometimes "relevante" means that, but often, as here, it means important.

One or two further suggestions, if I may. For "a estas alturas" I would say "by this time", and for "obras de entretenimiento" I think "works of entertainment" would be better than "entertaining works". "Literatura de entretenimiento", a concept Cervantes addresses in the prologue to the Novelas ejemplares, really means more or less prose fiction.

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Note added at 1 hr (2019-04-29 21:53:24 GMT)
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Actually "contrary to" might work, I think.

The idea is similar to "despite", but it's not quite the same, and I'm sure that if the author had meant that he/she would have written "a pesar de" or "pese a".

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Note added at 12 hrs (2019-04-30 08:55:16 GMT)
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On the question of "naturalness", which Carol has raised. To me it's a high priority; we are always aiming to make our translations read like originals. At the same time, we are (or should be) aiming to be as faithful as possible to the author's intended meaning. There can be a trade-off. Assessing "naturalness" is subjective; if a possible rendering jars, you have to find an alternative. But our job is not to write what we ourselves would ideally have said in the context; it is to find a solution that is acceptably idiomatic and at the same time accurate. I believe my proposal meets that requirement. In my view, "naturalness" can be too narrowly conceived as what amounts to cliché, phrases that are commonplace and endlessly repeated. Now that Internet searches are so quick and easy we tend to google what we're proposing to say to see how many others have said it before, and the more we find the more "natural" we consider it. But I think this can be a fallacy.

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Note added at 1 day 1 hr (2019-04-30 21:27:02 GMT) Post-grading
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Thanks, Linda. That's a generous decision. I hope Cecilia doesn't feel hard done by.