I recently finished reading The Metamorphosis by Franz Kafka in the original German. It proved to be quite a chore, but along the way my understanding of the German language improved greatly. One big reason for this improvement was that the version I read was an Interlinear Book.
The concept is pretty straightforward, and not much different from the side-by-side "readers" that people have been using for ages. When I read Pinocchio in the original Italian, it was also a dual-language reader, with the Italian on one page and its Englinsh translation on the opposite facing page. This is a very useful tool for the language learner because it allows you to stay in the context, but to find a good translation when you get stuck.
What Interlinear does differently, though, is that each word or phrase is accompanied by its translation in a smaller font, directly below.
The smaller font for English helps it to stay out of your way while you read, and having the words right there for translation makes them easier to find, so that you don't have to switch to an English page and try to figure out where in the context you can find the word or phrase you need. It also helps because phrasal concepts can be grouped and translated without forcing you to try to parse an entire sentence to understand how it's different than the word-by-word translation.
This does not come without a small price, of course. Different grammars don't always line up squarely. In German, for instance, the prefix to a verb gets moved to the end of a sentence, so at times it might be unclear why a verb's translation is different until you examine the entire sentence and see no translation underneath the prefix at the end of the sentence.
It can also, at times, leave the English translation arranged in a way that makes no grammatical sense. You're not going to read the lines in English and enjoy the book. But I believe this is actually an advantage, because it prevents you from getting comfortable reading long passages in English, and instead it forces you to continue the journey through the original language of the text.
The books are available in .pdf and (experimental) .epub versions. I used the .epub version and read on my iPad Mini, because I just don't have the patience and attention span to sit and read something in PDF.
For the most part, this actually worked out to be pretty great, although I noticed a few occasions where the text wouldn't line up perfectly, leaving a German sentence at the bottom of my screen and having to turn the page to see the English translation that should have been directly underneath it.
Example: The end of one page:
And the beginning of the next page:
While this was a minor nuisance at times, I think I actually began to like when this happened, because it was that much more motivation for me to try to figure things out rather than immediately cheating my eyes downward for a translation.
I think this is one of the challenges an e-book author faces when trying to create a somewhat formatted text inside of a dynamic format like epub, and I would say that for the most part, I was pretty impressed by how well Interlinear kept things together. One separate frustration I noticed, unrelated to Interlinear, was that the iBooks app seems confusingly unresponsive to screen rotation, which I eventually solved by disabling rotation when reading.
Overall I was pretty pleased with the Interlinear product. I've long stressed that the best way to improve your skill in any language is by using it, and this is definitely using it. Reading something in its original language — especially something on the writing level of Kafka — is a workout, and brings several challenges and opportunities to learn.
I've made great progress in language learning when using bilingual readers, as they give you access to common turns of phrase and expressions, along with a human translation that retains intended meaning, rather than a machine-generated word-by-word translation, and my experience with Interlinear was very much a good one, particularly as it finds a creative way to bridge the gap between side-by-side translations and e-books.
When I first started reading, they were offering books for four languages. At the time of publishing this, they're up to seven. Hopefully there will be many more coming, because I'd love to do this in Polish next. And I recommend you have a look for whatever language you're learning.