I've read about how the word 'aisle' and 'isle' each came from the French 'aile' and 'ile', respectively. I also read how the there was confusion between the two words, such that when 'isle' gained its 's', 'aisle' soon followed. Does anyone know the full story?
Here is the discussion of island and isle in John Ayto, Arcade Dictionary of Word Origins (1990):
island [OE] Despite their similarity, island has no etymological connection with isle (their resemblance is due to a 16th-century change in the spelling of island under the influence of its semantic neighbour isle). Island comes ultimately from a prehistoric Germanic *aujō, which denoted 'land associated with water,' and was distantly related to Latin aqua 'water'. This passed into Old English as īeg 'island,' which was subsequently compounded with land to form īegland 'island'. By the late Middle English period this had developed to iland, the form which was turned into island. (A diminutive form of Old English īeg, incidentally, has given us eyot 'small island in a river' [OE].)
Isle  itself comes via Old French ile from Latin insula (the s is a 15th-century reintroduction from Latin). Other contributions made by insula to English include insular , insulate , insulin, isolate [via Italian) , and peninsula .
Meanwhile, Ayto offers this comment on aisle:
aisle  The original English form of this word was ele. It was borrowed from Old French, which in turn took it from Latin āla 'wing' (the modern French form of the word aile, has a diminutive form aileron 'movable control surface on an aircraft's wing' , which has been acquired by English). Besides meaning literally 'bird's wing,' āla was used metaphorically for 'wing of a building,' which was the source of its original meaning in English, the 'sides of the nave of a church.' The Latin word comes from an unrecorded *acsla, which is one of a complex web of 'turning' words that include Latin axis, Greek axon 'axis,' Latin axilla 'armpit' (whence English axillary and axil, and English axle.
The notion of an aisle as a detached, separate part of a building led to an association with isle and island which eventually affected Middle English ele's spelling. From the 16th to the 18th century the word was usually spelled ile or isle. A further complication entered the picture in the 18th century in the form of French aile, which took the spelling on to today's settled form aisle.
Glynnis Chantrell, The Oxford Dictionary of Word Histories (2002), and Eric Partridge, Origins: A Short Etymological Dictionary of Modern English (1966), offer similar accounts of the relevant words, but Ayto's discussion is the most detailed and most readable of the three.