Alex65 wrote: ↑
Thu Jul 27, 2017 8:28 am
But for sight reading practise I have to study the piece until I'm convinced I can played it right at once, right?
That's not really what sight reading is. Very few (if any) guitar players can play an advanced piece perfectly just from sight reading it.
What you can do to practice, is to take fairly easy pieces (for example from volume I-III here at Delcamp) and "look ahead" as far as you can when you play (for example, read the first bar, then play it while you read bar 2). You may need to start with only a note or two ahead. This is mostly for practice to get used to remembering the notes and become more fluent. When I play "new" pieces I don't really look ahead much at all, but you need to be able to read fluently without thinking. If you are able to look ahead a few notes, you may have enough time to figure out more difficult notes before you reach them.
Sight reading is basically the ability to read the notes and play them back (not "perfectly", that's more a musicality issue which depends on practice and preference).
There are also a couple of decent books on this topic called "Sight reading for the Classical Guitar" by Robert Benedict which have been recommended by many.
For an enjoyable input on mastering sight reading, see this article from Classical Guitar magazine:
http://classicalguitarmagazine.com/5-ti ... or-guitar/
By the way, you should not have to invest in anything, not even books, all you need is sheet music and dedication.