A book shelf speaker to go on a bookshelf.

Not sure how much space you have, but I had a very good expereince with the B&W 602 S3's. They are front ported which enables them to sit real close to a back wall. For a monitor they have a very natural sound across a broad spectrum. Decent price, especially if you can grab some used. There's also a matching sub that you could mate later on if you wanted to.

I also tested the Acoustic Energy Aesprit for a month, Phenomenal speaker with unbelievable bass output.

Good luck.

If you can find a less expensive, used version of the classic LS3/5A from Spendor (or another British manufacturer with the BBC license to produce this model) you'll be pleasantly surprised at the high level of listening enjoyment.
Mine are positioned at the front of the bookshelf edge, and you can experiment with support, such as AudioPoints, or Aurios, or Mi-Roller. Great for nearfield listening since the sound image snaps into focus amazingly close to the front of the speakers, and does not have deeper bass, so not as wonderful listening far out in my room.

Some Linn's. Thiel SCS's.

I've tried a lot of bookshelf speakers, my preference is Totem Rokk monitors. No longer in production, you should be able to find a pair for less than $400 on the used market.

Allison has a wonderful speaker designed expressly to sit on a shelf. Has excellent bass response. It is not inexpensive; think it is around 2500/pr.

Linn Tukans do well on a shelf, too, and placed near a rear wall as they were designed with this in mind. The Tukan is one of the better true bookshelf sized moniters out there and s/b about 500/pr used.