List of resources
Is there somewhere a list of useful resources (books, web pages, Youtube-channels, ...) for the Klingon learner?
tlhIH Satlho' jIH!
I can't think of a single place that lists all the resources that are out there. But here's a listing of most of them...
The Klingon Language Wiki has a pretty good page about all the canon sources (i.e. from Dr. Marc Okrand): http://www.klingonwiki.net/En/PortalCanon
The Klingon Language Institute has an online course designed to help students pass the Level I Certification test. This Duolingo course would also be very helpful, but is not specifically designed to match up with the test requirements so teaches some things that are not on the test and may skip some things that are on the test. The KLI course is at: https://www.kli.org/members-only/klcp-prep/
The KLI has also produced a number of books with side by side English and Klingon. Most of these are not currently in print, but can sometimes still be found for sale and may be printed again if there is enough demand: https://www.kli.org/activities/kli-press/
There are two YouTube channels that provide good opportunities to hear good pronunciation and practice some Klingon:
The Klingon Teacher from Germany: https://youtu.be/auqS6FR_RDE
Klingon the Easy(ish) way: https://youtu.be/YjROGAY19pU
Finally, here's a few music videos just for fun. Look around for more!
I knew I was forgetting something. Until the iOS version is finished or if you have any other problems with boQwI', https://hol.kag.org/ also has some excellent tools including word look up and pronunciation (it actually uses dlyongemallo's TTS, so don't try listening for a different pronuncation, you'll just hear him say it in both places).