Ok, so I completed the whole defeating the original dragonborn, Miraak, but when I went to read the book to get back to Solstheim, it keeps saying "You can't concentrate on what you are reading". In the end I just got myself killed and went back that way instead, where I was able to complete the quest.

I'm pretty sure it's supposed to take me back, though, via the book. So I'm thinking I missed something out. Any help?

have you choosen a power ? i dont quite remember the quest since i only did it once and was when the game was released for pc. but yes the book should take you to Solstheim or maybe some mod Conflict?

All the perk trees appeared in a circle around the book, it just gave me the choice to get rid of some of the perks and get perk points back to add to something else instead. (Incidentally, I took from Alchemy and gave to Two handed, which is my character's strongest point) but still it kept saying "You can't concentrate on what you are reading". I thought it may have been a bug, so after waiting until I got nearly frustrated enough to just give up without saving (!!!), I stood in the black water (acid?) and ended up in Solstheim that way. I was worried I wouldn't be able to complete the quest (or worse, have to do it again!), fortunately though I completed it.

EDIT: As for mod conflict, could be. I share the PC with someone who's installed a few dozen Skyrim mods. Recently it was 'improved' and has been freezing more than ever, so mod conflict is definitely a possibility.

You don't need to choose a power to return to Solstheim, usually. The 'power' you gain from the book on the summit of Apocrypha is the ability to clear all your perks in a skill tree, but you don't need to pick any. I honestly can't think of a reason why you shouldn't be able to return. You did read the book in the center of the stage right? Not the one in your inventory?

Otherwise it's probably mod interference, or a glitch.

Yes it was the one in the centre. When it didn't, I then checked in my inventory anyway. Just in case. No change. Ah, clear a tree, that 's right. I only had one perk in Alchemy so put it in my strongest perk tree anyway.

I guess it must just be a bug if I didn't miss anything out and if noone else had this problem.