How ide is the bookshelf? If lining up studs with the edge of the bookshelf is too difficult/just not happening, then you could get a piece of timber and the bridge the studs. Basically, you find ths studs(the vertical pieces of timber in your wall) screw a length of timber into two of them and then secure your bookshelf to them. One advantage is that if your bookshelf is, say, 1200mm wide and your studs are on a 450mm spacing, then you can pick two studs 900 apart, screw a piece of timber between them then lock your bookshelf in to that and you shouldn't be able to see it (since the bookshelf should cover the piece of bridging timber)

Good luck with it