Mountain Guides
Mountain guides can take two general shapes, as they can be either people or paper guides, equally set to assist you get up and down the mountain of your choice. Both should be of quality construction to get you up and down safely, though which you select will have to do with both your own experience and your level of confidence when setting out on your expedition.
Paper mountain guides can take many forms, from little government-issued pamphlets about a certain climb to handwritten remarks from friends to full- length, pitch-by-pitch guides. They all have their own strengths and weaknesses. Although a book presenting full route descriptions, comprehensive maps, and stories of past ascents may be a precious tool when climbing a mountain, it's easy to rely too greatly on these sorts of mountain guides. Heavy snowfall, avalanches, or rock falls can change mountain faces considerably from year to year, and although the guide may be up to date when it was published, each year after publication makes it harder to trust. Also, don't think you can climb a mountain by relying on the guide through the whole climb; you should have the skills and experience to climb the same mountain with no guide and get to the base safely.
Smaller, less formal guidebooks or even handwritten notations written by climbers who have done the climb before can be a great benefit to climbers who are looking for up to date information about a route. Although these guides won't provide you step-by-step instructions, they will generally stress any major dangers about the route or less-than-obvious characteristics that you should keep in mind. Frequently these sorts of guides, in addition to full-length route guides, are the top combinations for those who have not climbed a certain route before.
Finally, in-the-flesh mountain guides are by far the safest and most reliable way to climb new mountains and new peaks. These guides, either those hired or contacted through friends, ought to know the mountain and route you're ascending and should have completed it before. Mountain guides, particularly those hired through a trustworthy guide service, will not get you to the summit if you don't have the alpine skills to do so. Though, as long as you have the physical ability for the given route they'll get you up and down in safety, putting your well being before any other consideration on the trip. Granted, they'll be more pricey than a paper guide, but if you're new to mountaineering or doubtful about a particular peak they're by far the finest way to climb.