Check out this page of the LearnGaelic site. It goes over the different sounds that letter combinations can make depending on the location of them in a word. I've been using it in conjunction with this site, and my spelling and hearing recognition has improved. https://learngaelic.scot/sounds/index.jsp
Yes, both are valid. And the one you would expect, if you have read a pronunciation guide, for a broad gh is the one way you can't say it.
The y-sound you describe (i.e. a slender gh) is sometimes heard in some words with a broad gh, either as a variant or, in words such as brèagha, as the standard pronunciation.
But the v-sound is the most common pronunciation for this word and is unique and intriguing. I have always assumed it was caused by confusion with the word leabhar 'book' since they probably had nothing to read other then books when reading was first introduced.
Both words are of Latin origin, introduced by monks bringing the Bible. Ironically, the bh in leabhar is usually pronounced as a /w/ not a /v/ these days though both are possible. D