Bookshelf speakers that sound good on a SHELF or SIDEBOARD

Vandersteen VLR designed for exactly that instalation , easy load, flat to something crazy like 47 hz and integrate very well with unique Vandersteen powered subwoofers, so you have some system growth options
read the reviews and go listen, made and designed in USA
and
KEF LS50
and
Sonus Faber
and.....

disclaimer
happy Vandersteen owner but i have other gear as well...

rega rs1 is impervious to placement; likewise the pricier paradigm sign s1

+1 for Rega RS1 , doesn't know how to sound bad .

Not a 100% written in speaker design laws.
But mounting on a shelf or sideboard, usually means your going to get some bass loading addition because of close proximity to walls, corners or the sideboard itself.
Because of this I would look more to speakers that are infinite baffle design (sealed enclosure)  not ported, as these will help keep the added bass bloom down.

Cheers George