What area you should be focussing on is something we can't tell without seeing some games you lost. If I had to guess, it's probably tactics, or otherwise just not knowing what to do (bad planning).
If you have good tactics books, keep doing the exercises until you see them all in a split second.
Perhaps a good next book would be a games collection with heavy annotations - the classic in this area is "Logical Chess: Move by Move" by Irving Chernev. The Silman book you mention also gets good reviews.
Think like a grandmaster is OK but it's very abstract, it is hard to apply the ideas to practice.
But the hard advice stays: books don't improve your play. STUDY improves your play. You should first look at the games you lost and figure out why you lost them. Perhaps the problem is something else entirely (like bad concentration).
Can anybody suggest me what kind of books a 1400-1500 player should be reading? I just cant seem to get any better. I have a few books on tactics and one by Edmar Mednis on the opening. I have heard "Think like a grandmaster" by Kotov is good but is it too advanced? I have also looked at a book by Silman called "The Amatuers mind". Need some advice before i buy one. And in general which area should i be focussing on.