I would feel somewhat estranged by calling it the best - but it is the only covering the entire country (at least until the next is published - should be finished in one or perhaps two years). Unless highly experianced in Neotropical birds, you will often find it difficult to decipher many of the drawings and the text for each species is very brief (and voices are not described at all - only references to where you can find recordings!). Depending on the region you plan on visiting in Peru, a combination of guides is far better (in many cases you can just copy specific plates to bring along). For Pelagics, bring along a guide for pelagic birds, likewise for waders. For the high Andes (~above 2500 m. asl) the guide by Fjeldsaa & Krabbe - Birds of the High Andes - is unparalleled. However, this is a rather bulky and expensive guide, so you may prefer to have a look at a library and just copy a few of the plates relating to more difficult groups. For the Amazonian lowlands get "Birds of Ecuador" by Ridgley & Greenfield or "Birds of Colombia" by Hlity & Brown. The guide to Ecuador is good for northern Peru, too. For the Yungas in SE Peru, the best guide is a "Field Guide to the birds of Machu Picchu" by Barry Walker. SW Peru is well covered by Jaramillo's "Field Guide to the Birds of Chile". Otherwise we are looking at copies from a few plates of the usual "Birds of South America" by Ridgley and Tudor. Likewise, copies of a few plates from the most recent (vol. 8 & 9) "Handbook of the Birds of the World" will probably help, too. So, do get the Clements & Shany guide to Peru, but realize that the experiance will be problematic unless combined with other guides. I know above may seem "over the top" - but you are likely to only visit a relatively small part of Peru and as such only a few of the additions mentioned above are needed.
Good luck - Peru is World class birding.