Follow Jennifer’s advice if you want a detailed map for both Shetland and Skye. Look at the Ordnance Survey Landranger maps . They are at a scale of 1:50,000 or 1.25 inches to 1 mile. They show all roads, tracks and footpaths as well as marking places of interest.
Either buy once you arrive or buy on line from Ordnance Survey.
Being a large scale does mean you need to buy three maps to cover all of Shetland. Maps 1,2 and 3
Skye needs 2 maps 23 and 32. These miss a small part of Skye from the end of the Skye Bridge but you can probably get away without this section as long as you have e road atlas covering Scotland.
The Explorer maps referred to by Skyegirl are at a scale of 2.5 inches to 1 mile. The detail on them is fantastic and they are used by all serious walkers, but you are going to need several to cover all of Skye.
My preferred Road Atlas is AA Graet Britain, which come at a scale of either 3miles to 1 inch or 4 miles to 1 inch.