Always have trouble with your vs. you're? Their, there, and they're? It's and its?
Like it or not, common misuse of these words, misspellings of others, unnecessary or omitted commas, and general poor writing looks really bad to potential employers, professors, and even fellow bloggers.
But Marsha Sramek's The Great Grammar Book is here to help.
A primer on basic grammar concepts, the book is packed with examples, reviews, and tests (and, of course, an excellent index). Sramek has gone out of her way to make those examples interesting for adult writers and to not sound pedantic.
As a bit of a grammar nerd, I really appreciated the thorough, accurate information included, but I also liked the chapter on good writing principles. Because even if you get all the grammar right, it can still be difficult to write well.
The Great Grammar Book is by no means a final stop for learning about grammar and writing, but it is, indeed, a great start.