While we encourage you to peruse our miscellaneous spelling section, we strenuously suggest you don't take a miscellaneous approach to spelling. Constructing your own spelling rules and phonograms might be fun, but doing so will severely limit your children's ability to communicate. If this is your plan we can't stop you. But don't blame us when your kids try to spell "psychiatric" and fail.
Seriously though, spelling bears a systematic approach. That doesn't mean, however, that you have to treat it like its own separate subject—many advocate an wholistic approach to language arts, meaning that topics like spelling and vocabulary are included in broader studies like reading and writing. If children see the correct spelling of a word enough times, eventually they'll know it (or so the reasoning goes).
Spelling is important in both directions: students need to be able to read correctly spelled words in order to understand what they're reading, and they need to be able to spell words correctly on their own in order to communicate effectively. So whether you cover spelling along with other language arts subjects, or teach it separately, we hope you'll agree that teaching kids to spell properly is essential.
Most of the books you'll find in this section are meant to reinforce or fill gaps. Worktexts have kids write words over and over; reference guides remind them of the phonics rules. A few books are written primarily to parents and teachers, but the important thing is that the kids know the rules, remember them, and are able to use them in their own writing.
Don't freak out if your six-year-old can't spell perfectly yet. There's plenty of time to get him or her up to level, and one of the best ways to do so is to not stress out about it. Also, take an organic approach: when your kids are reading (anything, really, from history to comics), stop them occasionally and make sure they know why specific words are spelled the way they are.
Or make spelling lists that cater to specifc problems they may be having; or point out signs as you're in the store or driving; or have your kids help compose the shopping list, making sure they spell each word correctly. There are a billion ways to turn kids who can't read into excellent spellers, you just have to be open to alternatives to the workbook routine. Kids learn best in real life, and since spelling is a real-life skill, what better way to teach it than through their everyday experience?