This is actually an example of the Subjunctive mood in English. It's rarely used in everyday modern English speech, but you will encounter it occasionally in literature.

This is an alternate form of a type 2 conditional. The word "if" is dropped when subject inversion is used. A google search for "subject inversion" will give you plenty of details. If I was/were to let it [a wordless gasp], it would turn into a shriek. Were I to let it, it would turn into a shriek. ... a wordless gasp that would turn into a shriek of shock were I to let it ...

It means ' If I were to/would let it.' The word order is rather formal and poetic. It's not used in everyday English. Rather than focusing on uncommon word usage, I suggest you work on learning the basics like correct puncutation, spelling, and grammar. 'I' in English is always capitalized.

This is "Old English" so it's extremely rare to hear someone speak in this context. I would interpret it as "if I let it" tambien.