Assistance animals are animals (primarily dogs) which are specially trained to assist individuals with disabilities. The most well known in the UK are Guide Dogs for the Blind and Hearing Dogs for the Deaf, but you may also come across Dogs for the Disabled.
Assistance animals are extremely well trained and always allowed into libraries with their owners. If you are talking to someone with an assistance animal, always remember that the animal is working and should not be touched or petted.
If you think the visitor or reader with the assistance animal needs additional help, please ask them and they will let you know. Never assume that you know what it is they require.
If you are interested in knowing more about guiding people with visual difficulties in particular, there is some helpful information from the RNIB about Guiding People with Visual Problems. As well as 10 Useful Tips on Guiding from Action for Blind People.