[Advaita-l] Bija mantras

[Santosh Rao]

Namaskara,

What is the function of bija mantras (kleem, kreem, shreem, etc;) as per
smartha sadhana? Are they recommended or considered an impediment to jnana?

I've been reading that chanting these bija mantras, particularly those
associated with goddess Kali can be dangerous if done so the wrong way.
Should these mantras be left to the shaktas?

Also if anybody can recommend me some books on Shakti/devi worship I'd
appreciate it. Thanks.

Santosh