Wool 'soakers' are an option when considering reusable, washable nappies. The principle is that the wool garment forms an outer layer which is water repellent, rather than waterproof, so it remains breathable and soft to the touch. The soaker (a misnomer in many respects) can be considered as an overnappy. If sufficient absorbency is in place in the 'inner' nappy, then the 'overnappy' soaker should (theoretically!) only need airing between uses, rather than washing as it will not get wet. When the soaker no longer smells nice after airing, or it has picked up grime, it is time to wash it. You will also need to periodically retreat it with lanolin. If your soaker gets soiled by accident during a nappy change, then wash it immediately.
Soakers are generally knitted though some are felted wool. It is important that the yarn chosen is as high a wool content as possible, ideally 100%, as otherwise the other material may interfere with the special properties of the wool once it has been treated with lanolin. Lanolin is the natural waxy substance that sheep produce to coat their fleece to keep themselves dry. You may have also used a lanolin product when starting to breast feed to salve your nipples... but I digress!
Wool soakers are best hand washed due to the delicate nature of the wool. Although machine washing is a possible option on a wool cycle, it would be wiser not to take the chance given the infrequency and simplicity of washing by hand. Use a proprietary wool wash, or a suitable natural soap product, using only as much as you need to clean the item, without removing all the lanolin. Gently squeeze the soaker, rather than rubbing. Rinse thoroughly in warm water, and then blot dry by wrapping in a (clean!) towel to remove excess water, just like you would your very best sweater. Once damp rather than wet, you can dry flat in an airy place. Avoid direct heat and if you must line dry, do so only when you are sure there is only dampness left so that the weight of the water doesn't stretch your soaker. You will ideally need a minimum of three for full time use; one being worn, one being aired, and one ready to wear.
To treat with lanolin requires either a commercial product or else the use of pure lanolin added to hot water to melt it. For a few items, a teaspoon of lanolin melted in half a cup of boiling water with a couple of drops of wool wash, stirred into a gallon of warm water will create a dispersed solution. You can scale up or down accordingly. The addition of a small amount of wool wash to the boiling water helps the lanolin to disperse. Submerge your items overnight and then rinse gently, and dry as above.
Wool soakers come in several styles, but 'longies' are popular as they are warm and snug in colder weather, and can replace the need for separate trousers or leggings. They can often look quite quirky and distinctive, and fit with a 'handmade' look that is very fashionable. Several brands are available, but most are made by following a knitting pattern or else ordered from a Work At Home business.
It is worth trying a wool soaker before you buy them, as they are quite expensive and you may not like the look of them on your baby, or how you use them in combination with a prefold or fitted inner nappy. You could consider a trial of soakers alongside a variety of washable nappies through a trial service such as ThunderBots Nappy Trials.