So I'm only 3k, but every time I come back to these books I feel like I'm reading something completely new. Do these books ever lose their benefit? At what rank would someone have a exhausted them? Have you?

I've felt the same way (so far reading them at 10k and now rereading them at 6k), Andd, but I'm surprised you chose to put 38 Basic Joseki under Lee Sedol's pillow - that's actually the only book in the series I haven't found awesomely useful.

I would find it difficult to believe that your average shodan could explain "The Endgame" cover-to-cover. That book is crazy hard.

Agreed. Some understandable (for the average 5-10k) book on endgame is still needed. Also that stuff with half, third, fourth points, that's all Greek to me.

As for the complete series, I'd say that not each of the books is on the same level, hence the question of the OP seems a bit useless.

Karaklis, I simply don't believe that detailed endgame study is all that important for players below 5 kyu. Instead, just remember the "three rules" from Guo Juan:

1. Your sente moves are yours, don't let your opponent have them!
2. Try to take just a little of your opponent's sente. More is not possible.
3. Count!

_________________
My name is Gijs, from Utrecht, NL.

When in doubt, play the most aggressive move

I agree on the fact that the books don't seem to have the same value. Tesuji, Attack & Defense and Life & Death are praised everywhere. You hear very little from the others. I haven't read neither 38 basic joseki nor the endgame but I didn't feel like I was learning much from "in the beginning" (not to say that I understand everything that is in it but I get out of reading it without much in head: books like opening theory made easy or fundamental principles of go have brought more to my fuseki. Maybe I need to re-read it...
I'm 4k KGS at the moment and clearly, I would benefit a lot from re-reading Tesuji or going through L&D again (didn't manage to read it entirely yet... It is very very dry to my taste). I'm reading A&D as we speak (which will take me about 2 months in total as I'm repeating every single diagram of the book) and I love it. I like its very structured approach (maybe what is somehow missing in "in the beginning".
I'm starting to get a pretty indecent library in terms of books owned (especially considering the small quantity I've read) but the more I progress, the more I believe you can learn on this serie alone + some problem books. Reading a go book once is hardly sufficient : even from basic books, you still learn from re-reading them.

I had the same experience with "In the Beginning." Coincidentally, it's one of the books that I'm currently rereading, and I'm getting some nuggets out of it this time. However, back in the DDK days I read "Opening Theory Made Easy" a half dozen times, and it was simply fantastic. It was exactly what I needed then, and so well written that it was fun to read.

Since I've seen 6d players reviewing Davies L&D, it is hard to say what the upper limit is.

"In the Beginning" was my first go book. At that time I was around 16k and found it great.