Did you fully absorb the books or did you just read them? I have a feeling most people don't quite get all they can from books.
As a ~1780 correspondence player here I'm getting quite a bit from working though Silman's RYC (which was the first positional book I bought, not long ago).
But as I've read nothing else I can't really recommend anything new (other than mainly focusing on tactics, but I'm sure you've heard that before).
Silman's books are good for begginer's but now you need some good books.Pachman's 3 books "Complete Chess Strategy" are excellent.On these books there is everything a chess player should know.Read them again and again.
If OP decides to listen to you , he is screwed but it's his choice.I suggest to him to learn who Pachman was and then compare with Sillman.
p.s. Pachman's books are middle-game books, can't be outdated.
So, this isn't really the answer you are asking for but you can take it or leave it - it's free!
If I were you I would seriously re-consider wanting to learn more about positional ideas and purchase more positional books. I think with the Silman books you have really a lot of material to be mastered - you should be doing the excercises he recommends playing a sort of "solitaire" chess and writing down everything you see about the position and using his principals until they are truly mastered - that should take some time.
How does your set of tactics books compare? Unless you are spending a good portion of your time looking at simple tactics and combinations I suggest you reconsider how much of your study time you want to spend on positional ideas.
I suggest you do this - go back and review your last 5 losses and see where you went wrong. I'm fairly certain that it was because you dropped loose pieces, fell prey to a simple tactic, etc. I seriously doubt it was because you were dominated by some sublte positional nuance.
If you aren't familiar with Dan Heisman I suggest you look him up via google and read some of his ideas. Just for what it is worth, I have played chess off and on for about 15 years now and I am really not much better than you are - my rating is very similar. However, for much of that time I thought the way to get better was acquiring more and more knowledge from books and read a lot of material on strategy, position, etc. and it does nothing if you are still losing material. In short - spend some time looking at why you lose to see how you should improve!
I guess if you have read this far, I'll give you something to think about for positional chess books! Take a look at Secrets of Modern Chess Strategy by Watson and see if it is in line with what you are looking for.
Best of luck on your path to improvement!
This is a very good read:
Don't be put off by the fact that this was written by a 15 year old. It's very compelling and well written!
oh my........
Oh micheal the well of ignorance is deeper and drier than whatever container of water you possess 
People who dont want to read the classics, oh well...
The worst is that someone that needs to find the right book to read , won't , because a guy , self-called chess teacher, considers Groten and Helsten(who?) better than Ludek Pachman ,and probably from Kotov and Keres too(I guess they are outdated too).
If anyone knows where Pachman's 3 volume series Complete Chess Strategy can be obtained please let me know. I am not interested in the abridged 1 volume version which I own. To OP Pachman's chapter on rook play alone is worth the price of the whole book.
I agree with Aidin299 and his thoughts on books and tactical work is a bigger priority. I want to clarify a few of the books though. My feeling is that most players try to learn too many strategical ideas and forget that the goal of strategical ideas is to help lead to the creation of tactical ideas. Both of the first two books recommeneded by Aidin are excellent (along with Silman's book, his newer edition is great and he finally got it that amateurs cannt really grasp the refined ideas in his early editions.
1-chess strategy for club playes ( Groten)........2- mastering chess strategy ( Helsten).
I would also add that players should work on typical strategical ideas that occur in their openings. Players under 1500 should focus 100% on the central struggle unless there is a very clear concrete reason to do something else. Avoid blocked central stuctures like the plague. Play a large variety of openings.
I also would say that the pachman books are excellent but the writing style is a bit hard to work with. The newer strategical books are easier to digest and understand but if you ever get a chance to read the pachman books cover to cover you will have a massive base. I really liked gooten's book because it gives a thinking process and the newer helsten book is nice too.
The Dvoretsky and Aagaard books are more difficult and should be used at the earliest of 1800.
If anyone knows where Pachman's 3 volume series Complete Chess Strategy can be obtained please let me know. I am not interested in the abridged 1 volume version which I own. To OP Pachman's chapter on rook play alone is worth the price of the whole book.
The first two volumes you can find on Ebay or Amazon fairly easily. The 3rd volume in hardback is very difficult to obtain and expensive if you do see it around. There seems to be some paperback ones on Amazon though.
(chess teacher) ?
Hello Everyone. I am a 1300 on chess.com but after reading Amateurs Mind and Reassess your chess 4th edition, and SIlmans complete endgame course. All three books by Jeremy Silman. I have beaten two 1400's and a 1500 on chess.com. Please what book should i read next now that i have a very basic understanding of posiiton. Also, what book do i read after those? Please help me