Malta had a great strategic significance for the British in the Mediterranean, as a result of its geographical position. Being in the centre of the Mediterranean, aircraft, ships and submarines based there could interdict any shipping trying to move past it. This proved to be especially valuable, as it controlled the shipping lanes from Italy to North Africa. The RN based two main forces there - Force K, and the 10th Submarine Flotilla. Force K was a surface force, based around two light cruisers, Penelope and Aurora. It destroyed several Axis convoys, before increased German and Italian bombing forced a retreat. The 10th Submarine Flotilla was based in the old lazarette on Manoel Island in Marsamxett Harbour. Composed mainly of small U-class submarines, it was highly effective at interdicting shipping to and from North Africa. The flotilla produced some highly successful submarines, including HMS Upholder, which sank ~120,000 tons of shipping, making it the most successful British sub of the war. It provided a base for the RAF to launch raids on the Italian coast, and for repairing ships damaged in the waters around it. The British carrier Illustrious, heavily damaged by Stukas while escorting a convoy to Alexandria, received major repairs in Valetta's Grand Harbour. Aerial reconnaissance from Malta also proved useful. For example, Martin Maryland recon aircraft from Malta were able to confirm the presence of the Italian fleet at Taranto before the RN launched their raid. Once the siege had been broken, Malta also provided an excellent base for offensive operations against Sicily and southern Italy. The Italians and Germans were, of course, trying to prevent the British from using all of these functions. As far as books go, the big recommendation is James Holland's Fortress Malta, which is an excellent popular overview of the siege. Bradford's Siege Malta 1940-1943 is also worth a read. If you can find them, the British official histories The Air Battle of Malta 1940-1942 and The Mediterranean Fleet will give a good overview of the British side of the battle.
this is a little off topic but at the beginning of Halo 2 the aliens are boarding and blowing up the human defense ships, the first one destroyed is called "the Malta" and the covenant quickly reach earth after that. I never made the connection to WWII.