A valuable event because:
You can see huge number of publishers (all at one place).
Publishers will display what they are about to publish (new products).
You can get key information for submitting knowing the new products.
Only place you stand a good chance to meet publisher, art director, etc.
You can get key information that you can't get any where else, and you can learn each publisher's strength, what they're looking for, and what they want from you.
Book fairs are like speed dating because you can get numbers and names as quickly as possible, make new contacts, and even might fall in love (with a publisher).
Maximize the opportunities while you are there:
Take notebook with you (write down ideas)
If you can take a camera...do! Ask before you take a picture.
Exchange ideas with colleagues
Can see what foreign publishers are doing with different subjects