Are there any books that were either written or translated into the Klingon language?
Quite a few!
And there are others, including comic books. There are more coming in the future.
A warning: the earlier books, like Hamlet and Much Ado About Nothing, were translated when we had a lot less vocabulary and understanding of the grammar than we do now. You may find reading them a bit difficult as the translator squirms and tries to put together some pretty complicated sentences to try to get across simple concepts.
I bought ta' puq mach, figuring a book destined to children should be a better starting point than Shakespeare. It is in tlhIngan Hol with a German translation on the side. Since I am trying to learn both languages, I thought I could use it as practice for both. It works, too, for me. The German is a translation of the Klingon, and not a translation of the (French?) original, so it too is kind of simplified.
Reading longer sentences, that make a story also gives a better idea about how the language is used in practice, and I think it is good practice. Of course I still need to look up quite a few words, and I am decidedly not to a point where the reading is enjoyable... But I hope I will get to that point eventually!
"The Little Prince" has recently been translated to Klingon and is available alongside the German translation under the name "ta'puq mach".
You can find a couple of different books on Amazon (Shakespear, of course) but that's pretty much the most accessible that I have come across.