1. Yes it's quite alright to mix herbs together. In fact, doing so can often make herbal spells, potions, teas, etc stronger if you use more than one herb associated with your intent. Many herbal teas that you buy are actually a combination of various herbs, if you didn't know. For example, the chamomile tea I buy for relaxation purposes has rosehips, hibiscus flowers, bergamot flowers, etc added into it.
2. I would recommend chamomile, lavender, basil, mint, and sage to name a few. I've found that they seem to be the easiest to grow and work with. However, you need to check your climate zone to be certain what herbs will do the best in your area.
3. I wouldn't recommend using herbal powders for teas. The powders clump and turn into a paste, which isn't something pleasant to have in your teapot or your teacup. I suppose if you put it in very fine cheese cloth it might work, but I think it would be best to stick to the fresh or dried leaves and/or flowers for the best results.
4. I would recommend:
* Herb Magic for Beginners: Down-to-Earth Enchantments ~ Written by Ellen Dugan
* Cunningham's Encyclopedia of Magical Herbs by Scott Cunningham
* Homemade Health - Home remedies your grandmother knew - Simple & effective treatments from the pantry by Anke Bialas
* The Complete Book of Incense, Oils and Brews by Scott Cunningham
* Homemade Health (Herbology at Home) by Anke Bialas
These are just a few good ones that I know of. If you have a kindle, I believe that you can get the books by Anke Bialas for free, or at least borrow them if you are an Amazon Prime member.