I'm searching for a group that translates books into Esperanto and would like to volunteer some of my time to help in the endeavor of translating. Duolingo has an immersion feature that allows several people to translate articles in a group and I am wondering if something like that exists for translating books into Esperanto.
If the book is under a free license or in the public domain, you may translate it on Vikifontaro. The project aims to "provide an open online universal repository of free texts and primary sources, collaboratively curated by volunteer communities in all languages, integrated with Wikipedia, Wikimedia projects, and other sources, reusable by all".
Translation activities of this works from one Language to an other are also welcome. There is a community, and a huge infrastructure. Mediawiki, the software powering the website, do have extension to ease translation (you can use them on Wikipedia for example), but I don't have information about it's installation and support in the case of Wikisource. You can consult the more detailed information about translation inclusion on the project.
I have never heard of a group like this, and there are a few reasons why I wouldn't expect to.
As has been pointed out, translators often work alone. Roughly 10 years ago, I produced the Esperanto content for the DVD "Flying an Octopus" and in addition, I solicited help from others for different individuals to translate sections of the subtitles into various national languages. I received criticism from some professional translators who felt that this approach would result in a translation that was not stylistically consistent throughout, and that different technical terms for example might be used for the same thing in different sections of the translation.
Another group translation project that I am aware of was the translation of the first Harry Potter book. Much of the work ended up falling on a single individual anyway, and in the end they were unable to obtain permission to publish.
Finally, consider the state of "immersion" on Duolingo. Initially, this was the core of Duolingo's business model. They thought that they'd be able set up a crowdsourcing site for translation, and then make money off the translations. It's already been a few years since they've rolled out "immersion" to a new language, and it's clear that they're moving away from this and looking for a new model to make money. If Duolingo is moving away from it, I wouldn't expect other groups to move to it.
If you are a fluent Esperanto speaker with years of experience reading and writing good Esperanto, I would encourage you to translate and publish. If a whole book is a bigger project than you want, why not try something shorter?
Eldonejo Mistera Sturno calls itself "...projekto por traduki majstrajn verkojn de scienca fikcio, fantasto kaj hororo."
Yes, I run an email discussion group called verotraduko: https://groups.google.com/forum/#!forum/verotradukado
It's meant for books, films, videos, etc to do with exposing the causes of the current wars.
Konkrete, vi povus akcepti la proponon traduki al via gepatra lingvo la Esperanto-tradukojn de la eroj de la ejo ‘Epsilon-Teorio’, kaj poste afiŝi ilin, kompreneble. Tiu ejo, fare de ne-Esperantistoj, pritraktas monajn kaj investajn aferojn de Usono. Jen estas la ligilo al la loko de la Esperanto-tradukoj: https://ekonomiabultenopriusono.com/