Can I share my iCloud Photo Library?

Once I create an iCloud Photo Library, can I allow permanent access to another user, viz. my spouse?

Only if you tell your spouse your iCloud password. Then your spouse can access the Photos library using the www.cloud.com web page. But that would also give access to all your other personal data, Contacts, Calendar, Mail.

I would not do that.

It is better to share only selected albums as Shared albums. See: iCloud: iCloud Photo Sharing FAQ