There aren't that many species found in Nicaragua that aren't in Costa Rica as well, but there are a number of important ones. There is a Birdlife endemic bird area in northern Central America and Chiapas in Mexico that extends into the highlands of western Nicaragua. Such important species as White-breasted Hawk, White-bellied Chachalaca, White-faced Quail-Dove, Red-throated Parakeet, Pacific Parakeet, Fulvous Owl (I think), Emerald-chinned Hummingbird, Green-breasted Mountain-gem, Blue-tailed Hummingbird, Bushy-crested Jay, Rufous-browed Wren, Slate-coloured Solitaire and Blue-crowned Chlorophonia. There will probably be a few other species from further north in Mexico that extend this far south, but these are the important ones. Howell and Webb is quite big, but still worth taking, even if you leave it in the Hotel room. In the north west of Nicaragua Howell and Webb will probably be the best guide, but there will also be some birds not found (especially along the north coast and in rain forest). Howell and Webb is not suitable to use on its own in Costa Rica. There are probably hundreds of species that it does not have. In short, the most important guide for you is Garrigues and Dean, but Howell and Webb would be very useful in Nicaragua and Stiles and Skutch would be nice to have as well, if you have the space, because it would give you a lot more detailed information on the Costa Rican species.
There is almost no information on birding in Nicaragua (as you have probably found), so please consider taking lots of notes on where you travel to write a trip report for us here at birdforum.
Tom