One bit I really like for a number of reasons is the so-called Spanish Snaffle AKA Port-Mouthed Kimblewick, which essentially has a mouthpiece like a Port-Mouthed Pelham and slotted D-rings so you can set the level of curb action you want between very mild and mild. We used that on a horse who habitually got his tongue over snaffles (wasn't really comfortable in snaffles) and he was really happy working with that bit. Horses and bits can be a pretty individual thing, dependent on both horse and rider. The most important thing is to have something that's not irritating to your horse (not pinching the lips or too hard on the bars of the mouth or digging into the roof of the mouth or just an uncomfortable shape for a particular horse's mouth), and which allows you to communicate well and in a nuanced way.
Some horses are happier bitless (halter, bosal, hackmore, etc etc), which can be just fine especially for trails, pleasure riding etc. Sometimes that's just the way it is, often that's because people's hands aren't quite as soft with bits as they should be.
The best book I personally ever read on the subject of bits was by the late Australian horseman Tom Roberts, called "Horse Control and the Bit." It's a really informative, intelligent read studded with anecdotes and case studies, which also debunks quite a few bit myths. Although now out of print, Tom Roberts books can still be occasionally found on e-bay etc. I've read a lot of horse books in my time but his are amongst my all-time favourites for training horses, riding horses, understanding horses and getting the right gear for a particular horse to be comfortable.
Speaking great horse books, I also recently read "Horse Watch - What it is to be Equine" by Marthe Kiley-Worthington and thought that was a really interesting, very detailed book well worth recommending to others.