The issue is that although book club is a casual affair, it is still a social excursion so you don't want to look sloppy. Even if you are meeting in someone's home, you should still dress as if this were taking place at a coffee shop or bar so leave the yoga pants at home. However, the flip side of that is that you don't want to overdress because then you appear to have taken a wrong turn and ended up in someone's living room by mistake. Luckily, there is a very simple option that will work all year round and is probably already in your closet. Behold the knit wrap dress.
A knit wrap dress is flattering for pretty much every body type and is just as comfortable as the yoga pants I have forbidden you from wearing to book club. They are available in any color and pattern imaginable, as well as every price point. The fact that it is a dress makes it feel like you put more effort into your outfit than you actually did, but the fabric is relaxed enough that you won't look like you are dressed for the wrong occasion.
You might be thinking, "Why can't I just wear a nice pair of jeans and t-shirt? After all, no one else looks nice for book club." It doesn't matter if everyone else shows up in their pajamas because you should take pride in how your present yourself. Plus, if you don't want style advice then you are probably reading the wrong blog. I love jeans and tees as much as the next person, but to reach the same level of "stylishly casual" would require much more accessorizing. By suggesting a knit wrap dress I am actually simplifying your outfit.
While the sleeve length of your dress may change with the seasons, the basic formula for book club chic is always the same. Start with the knit wrap dress, add a pair of flat shoes (ballet flats, sandals, or riding boots), and throw a cardigan into your bag in case the venue is cold. In the colder months you might also want to add a patterned scarf and tights. It is really that easy. There is no worrying about adding a statement necklace to dress up a t-shirt or wondering if you should wear heels with your jeans. You are presentable without looking fussy and will be comfortable enough to spend hours chatting.