gentle parenting book recommendations?

My 20 month old had been pretty aggressive the last couple of days! I don't know what is going on. She pinches and hits me, and yesterday when a friend and her little girl were over, she pulled the girls pigtails several times throughout the day.
I have shown her "gentle, like this" and today I did a "time-out" for a minute. She fake cried and then snuggled against me being super cute, so that was the end of that.
I always tell her to "look with your eyes!" when it comes to kids ponytails/bows/headbands, all that jazz. so she knows better.
When my husband and I arm firm with her, she laughs in our faces.

Any advice or tips? I am feeling totally scared and unprepared for this part of parenting.
Thanks!

There's a ton of links on my page  

My basic belief is that there's a need behind every behaviour, they just can't meet it appropriately because of lack of experience and cognitive immaturity. So you can teach them to be self aware and to meet their needs effectively through positive interaction.. so you don't need to use negative interactions or fear at all.

My 2 fav books for this age group are
Unconditional Parenting, Kohn
Playful Parenting, Cohen

Good sites are the Natural Child Project, then click on articles, and
Attachment parenting international.

Time out isn't gently btw, it's using punishment rather than helping her understand why she did something and what to do next time. Punishment is about using control rather than supporting and teaching cooperation. An alternative if she pulled hair for e.g. would be to see why she was doing it, empathize, show understanding, explain why she shouldn't do that and then show what to do instead. 20 months is still really little, mostly the job is still yours as the parent to ensure safety boundaries. You can do that by being sure she's not tired, hungry, bored, overstimulated etc and that if she is finding something too difficult to understand or control an impulse, then you supportively guide her to play something else instead and move on.

Quoting Mama*AtoZ:" There's a ton of links on my page   My basic belief is that there's a need behind every behaviour, ... [snip!] ... too difficult to understand or control an impulse, then you supportively guide her to play something else instead and move on."

Thank you so much! That is a lot of super awesome info.
I know, I don't like idea of time-outs at all, I feel so mean  
I definitely agree with the correlation between a need and behavior. It just seems to have happened to fast over the last couple of days! I'm like, man, I need to read some books quickly, haha.
Sometimes just being reminded of why we parent the way we do helps.
Thank you so much, I will definitely check out the links on your page.