Reddit reviews Don't Just Sign... Communicate!: A Student's Guide to Mastering American Sign Language Grammar

I don't know of any websites, but yes, there are books on ASL grammar! This is a good place to start; it's by a non-native signer, but it's also affordable...there are some ASL grammar books out there that aren't! Here is an example of the expensive type. :)

To answer your question about active/passive hands...yes and no. It doesn't matter which you use, but it should be consistent. If you are right handed, then your right should be your "dominant hand" (DH) and your left should be your "non dominant hand" (NDH). If you're left handed, then your left is DH and right is NDH. As long as you don't switch back and forth all the time, you're fine. There are some people who switch for a few signs here and there (I have been guilty of it myself), but you're supposed to stick with one DH and one NDH. :)

Now, as for every half-turn of your wrists...actually that does kind of matter. I hope you don't mind if I explain the four components of a sign:

• Location - Where the hand is on the body. For example, in cat, the location is the cheek.
  
  
• Handshape - The shape of the hand, positions of the fingers. You can see most of the handshapes here, note that it goes beyond just the letters! For cat, the handshape is (usually) F.
  
  
• Palm orientation - Is the palm facing forward, back, left, or right? For cat, the palm is facing forward or "out" as most people call it.
  
  
• Movement - What does the hand do? Up, down, back and forth, in a circle, etc. For cat, the hand moves out from the face.
  
  If any one of these are changed, you're not signing cat anymore - in fact, if you change only the movement, you sign fruit! That's called a "twisting" movement. In the case of "every half-turn of the wrists" that would be palm orientation (PO). If PO is back (behind you), left, or right, you're not signing cat. So it does matter. Obviously if someone is sick, injured, etc they might make signs differently - but the way to be understood is to perform the sign accurately. There are slight variations from person to person, just like there are various accents in spoken language - but for the most part, everybody forms a sign with those four criteria intact.
  
  Of course, all that said...you can sign really really badly and people can figure it out. This video made the rounds among my friends a while back. The guy's signing is absolutely horrible but we all understood him. We wouldn't want to chat with him on a regular basis until he improved his skills...but we figured it out. :)

["Don't Just Sign...Communicate!] (http://www.amazon.com/dp/0984529446/ref=wl_it_dp_o_pC_S_ttl?_encoding=UTF8&colid=17NON4X8QY4C4&coliid=I2OM0S7SWCURT7) An ASL grammar book!

i , too, was surprised to learn that ASL was not a signed version of english, but rather its own language all together. i bought this book from amazon and it's very good as a primer for explaining ASL: as its own language with its own grammar, idioms, etc.

http://www.amazon.com/Dont-Just-Sign-Communicate-Mastering/dp/0984529446