Dragonbait
Explorer
There was once a monster manual pdf available on the old TSR site. You might be able to find it on the Wizard's downloads section.
To add to this comment
Caveat emptor when using any of the published adventures or the book on the awnsheghliens. Many of the world's rules were either ignored or not defined for some of the writers of these books. Awnsheghliens with psionics, races and monsters in adventures that were clearly stated as NOT being in the setting and then the whole magical item thing. If you are not that big on setting details then it wouldn't be noticeable but if you are a stickler for such things then some adventures would require revisions.
Finally, Birthright was supposed to be a low-magic setting (regarding the frequency of magic items and number of wizards), although not all authors seemed to have gotten that memo. So don't be afraid to remove magic items from NPC write-ups and such if it doesn't fit the flavor, and use monsters that can only be hit by magic weapons sparingly.
To add to this comment
Caveat emptor when using any of the published adventures or the book on the awnsheghliens. Many of the world's rules were either ignored or not defined for some of the writers of these books. Awnsheghliens with psionics, races and monsters in adventures that were clearly stated as NOT being in the setting and then the whole magical item thing. If you are not that big on setting details then it wouldn't be noticeable but if you are a stickler for such things then some adventures would require revisions.