Yarn for summer knitting
Wool is not the problem in summer; weight and air are. A fine wool tee can be cooler to wear than a dense cotton one. That said, plant fibres behave differently enough that dropping one into a wool pattern without adjusting usually disappoints.
Cotton
No memory and no elasticity. Cotton does not spring back, so ribbing goes slack, shoulders drop and a garment grows with wear and washing. It takes colour brightly, washes easily and feels cool against skin. It suits tees, tanks, bags and dishcloths, and anything where losing a bit of stretch does not matter. It suits fitted shaping that relies on ribbing much less well.
Linen
Stiff and almost wiry on the ball, and it softens with every wash. Linen has strong drape and very little stretch, so the fabric hangs rather than hugs. That fits loose tops and open stitch patterns. Knitting it can be hard on the hands, and wood or bamboo needles help more than metal.
Silk and silk blends
Silk adds shine and drape and almost no elasticity of its own, so it is usually blended. A silk blend gives you the cool feel and the shine while the other fibre carries the structure.
Blends do most of the work
Cotton with linen, cotton with silk, cotton with a small percentage of wool or elastane: blends exist because pure plant fibres give up elasticity. If you want a summer garment with shaping that holds, a blend is usually the practical answer. Read the composition percentages on the product page rather than the name of the yarn line.
Gauge and needle size
Plant fibres show uneven tension more than wool does, and they do not block out. Wool forgives; cotton and linen record. Swatch, wash the swatch, and measure it after it has dried, because plant fibres often change more in the wash than wool does.
Many knitters go down a needle size in cotton to keep the fabric from growing, and up a size in linen for drape. Start from the size on the band and adjust from what your swatch tells you.
Washing
Cotton and linen take machine washing better than wool does. They also stretch when wet and heavy, so dry them flat rather than on a hanger. Follow the care symbols on the band; they are the manufacturer's instruction for that specific yarn.
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Summer yarn, cotton, silk, or all yarn.