1) Trying to varnish a large flat surface with a brush is an exercise in futility; you - or at least i - cannot escape brush marks.
2) Trying to precisely cut-in varnish along a 2 metre length is equally futile; it won't end up straight … ever.
Following on from the above:
3) My new favourite thing in the whole world is a small foam roller for applying goss finishes. It gives an awesomely smooth coating and is seriously quick to use.
4) Masking Tape is your friend. It might take 2 hours to mask off the trim on 4 shelves, but combined with point 3 it takes 5mins to put a coat of varnish on a shelf instead of 30mins with a brush.
To explain the above - shortly there will be a post about my experience designing and building a bookcase, be afraid, be very afraid.