Are Grandmaster Repertoire Books worth it?

I just finished going through the Semi-Slav Repertoire Book by Lars Schandorff and it took a long time to go through. I'm was 1600 blitz on chess.com at my peak and do any other people actually read them or do they just use an opening database and  do it themselves?

I definitely recommend outside help when it comes to building your opening repertoire, whether it's videos/books or coaching, because it makes the process much more bearable. Coaches can explain the concepts behind why certain moves are played and can steer you in the right direction, which is significantly more difficult if you just go off computer recommendations and databases.

I'm a 1600-ish player, and I have one of the Grandmaster Repertoire books, on the Tarrasch, and to be honest it hasn't been at all useful to me. Club-level players don't play GM lines. In the Tarraxch, for example, a useful book for club players would devote most of its pages to the symmetrical lines rather than the approved kingside fianchetto variation, which, in my Tarrasch-playing days, I hardly ever saw.

These books are specifically written for very advanced players. I would say you should be at least around 2100 fide or 2500 blitz here to be able to make good use of these books. 

At 1600 studying openings is more about long-term progress and development since the vast majority of games are lost on tactics at that rating.  

There are plenty of good opening books aimed at the club player--and plenty of bad ones too! But when books are labeled "Grandmaster Repertoire," maybe that should be a hint that you are not the target audience the author had in mind!

Sure. I may not be at the level to properly utilize the Repertoire, I do find it useful for understanding the middlegame and when going through it, feel like I've actual accomplished something. Also it was from Lars Schandorff who helped me with the Queen's Gambit and Indian Defenses.

#1
"Are Grandmaster Repertoire Books worth it?"
++ No
Any book is obsolete while in print because games are played while the book is printed.
The books are worth it for their authors, not for the readers.
Authors never reveal their latest insights in their books.
The best book is not a book, but a data base.

I would not recommend it to people below 2000 rating, especially online. I have two of them - Avrukh's catalan repertoires. They don't really explain the WHY behind the moves a lot - because usually, grandmasters already know. That's why I dropped those openings. I could memorize the lines, but I wouldn't understand any of them.

Usually though, a grandmaster's perfunctory analysis is much better than us patzer deep analysis. When I was working through double bishop endgames vs b+n, the stuff my coach told me were all things that would have taken me a long time of dedicated study to find out, then I would have to test them, etc...

But he just told them to me casually, like, grass is green. Why would grass not be green when it's healthy? I'm sure he knows much more than that, except I'm not really ready for them yet.

I think opening books by everyman chess are best for the majority of us chess players.

I haven't finished any but Ntirlis is a very good chess author. He is a little more respectful to the club player and doesn't go into every single line that was played but gives you the fundamentals to learn the opening