Old English rædan (West Saxon), redan (Anglian) "to explain, read, rule, advise" (related to ræd, red "advice"), from Proto-Germanic *raedanan (cf. Old Norse raða, Old Frisian reda, Dutch raden, Old High German ratan, German raten "to advise, counsel, guess"), from PIE root *re(i)- "to reason, count" (cf. Sanskrit radh- "to succeed, accomplish," Greek arithmos "number amount," Old Church Slavonic raditi "to take thought, attend to," Old Irish im-radim "to deliberate, consider"). Connected to riddle via notion of "interpret."
Words from this root in most modern Germanic languages still mean "counsel, advise." Transference to "understand the meaning of written symbols" is unique to Old English and (perhaps under English influence) Old Norse raða. Most languages use a word rooted in the idea of "gather up" as their word for "read" (cf. French lire, from Latin legere). Sense of "make out the character of (a person)" is attested from 1610s. The noun meaning "an act of reading" is recorded from 1825. Read up "study" is from 1842; read-only in computer jargon is recorded from 1961.