I should mention that it's possible to worry too much about this. WIll speakers in the bookshelf you are describing sound as good as they would on stands ? Nope. Will they still sound "mighty fine" ? Probably...

I don't suppose you could post a picture of the actual bookshelf ? That makes a lot of difference...

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8