Buy or order thorugh the library a copy of "The Medieval Tailor's Assistant - making common garments 1200 - 1500" by Sarah Thursfield (Ruth Bean Publishers ISBN 0-903585-32-4)
older books which still hold good
and I also rather like "Medieval Costume and How to recreate it" by Dorothy Hartley (by Dover ISBN 0-486-42985-7 )for working clothes: her sketches are good without a heavy own-time overlay and includes photographs of her recreations on people
"Medieval Costume in England and France - the 13th, 14th and 15th centuries" - Mary G Huston - (Dover ISBN 0-486-29060-3) good reference - again with little own-time overlay, but be aware that French and English fashions varied greatly.
and to a lesser extent for an wider overview "Medieval Costume and Fashion" by Herbert Norris - (Dover ISBN 0-486-40486-2) as he applies a 1927 overlay more strongly than the other authors but covers civil ecclesiastical working and high status clothing. If you have an overview of the whole societal/status approach it makes pitching your own clothing easier.
All still available in reprint - some even second hand although I gather that the Sarah Thursfield is not easy to source at present.
The best ways to make up clothes for this period is
making a pattern on your own body, but if using ready made patterns I still make up a toile as a practice run for fitting and then take it apart and use that for the pattern. That is why I now never throw away any bedsheets or duvet covers - especially when working with good or expensive cloth
PLEASE NOTE THOUGH - English fashions are doing entirely their own thing during these periods, so don't just go and choose a random European look : influences alter by reign and diplomatic involvementhttp://www.reconstructinghistory.com/
- good range of their patterns stocked here in the UK by Paul Meekins http://www.paulmeekins.co.uk/patterns/patternsindex.htm
or Harper house to see the range available from many makers (by period) http://www.longago.com/
and the Greater Bay Area Costumer's Guild review section to see how easy the patterns are or aren't to work with http://www.gbacg.org/great-pattern-review/index.html