OnLocation(sm): Field Guide Tips
Some factors to consider when selecting a guide include the geographic coverage, the use of illustrations versus photographs, the physical size and weight of the guide, and several others.
The advantage to using a guide that just covers Eastern or Western regions is that many birds which are highly unlikely to be seen in your region will not appear in your guide, making it easier to identify the birds that are likely to be seen. It also means that the physical size and the weight of the guide can be minimized.
Good guides have range maps for each species (preferably on the same page as the species account, though this is less of a factor for Eastern/Western guides). These maps help you determine the likelihood of seeing a particular species based on location and season.
A good illustrator can emphasize details of a bird that are diagnostic in identifying a bird, thus simplifying the identification process. They can also convey the visual impression one will have of the bird in the field rather than what it looks like up close. Illustrations vary in the amount of plumage detail they convey. Excess detail can be confusing if the bird you are trying to identify is distant. Lack of detail may be a disadvantage if you have a difficult bird up close in a spotting scope.
Illustrated guides also typically employ plates where similar species are shown side by side and to scale so that relative comparisons are easily made.
Good photographs can convey details in color, plumage, and shape that are not accurately reproduced in illustrations. Some people find photographs easier to use in making comparisons with real birds though they seem to be in the minority.