It’s striking that two new books on the same subject – science’s current efforts to slow aging and lengthen human life span – view a single body of research through such different lenses.

In “Long for This World,” Pulitzer Prize-winning science writer Jonathan Weiner surveys the field as if from a mountain top: He’s intrigued, yet detached and skeptical.

In “The Youth Pill,” science and business journalist David Stipp hunkers down in the trenches with researchers as they test compounds, such as resveratrol and rapamycin and their chemical cousins, that offer the hope (so far based mainly on animal studies) of warding off many of the ills that afflict aging bodies.

Although no drug has yet been shown to extend the human life span, Stipp argues that such remedies are potentially just around the corner – and that the federal government should fund clinical trials to speed their arrival. Meanwhile, resveratrol (found in small amounts in red wine) is being marketed as a dietary supplement even though no studies have yet established whether taking large doses over long periods is safe and effective.

Weiner usually structures his books around the work and ideas of individual scientists, and for this one he has chosen Aubrey de Grey, a brilliant but eccentric computer scientist in Cambridge, England who has become an acknowledged leader in devising strategies to vanquish aging. As a protagonist, de Grey is unappealing: He’s good at seeing the big picture, but he’s described as an arrogant man who takes pleasure only in working, swilling beer and punting on the Cam.

Weiner uses their encounters to lay out current theories about why we age. Aging is not a biological constant: Some organisms (hydras and sponges, for example) seem virtually immortal, and even some closely related groups of animals (such as bats and mice) have dramatically different life spans. Human aging stems from progressive damage to cells and their DNA – caused by threats from within, such as dangerous byproducts of metabolic reactions, and from without, such as exposure to radiation or mutagenic chemicals. It’s also thought to result from inherited mutations.

De Grey simplifies aging to a list of the “deadly things” that eventually happen to everyone: the accumulation of junk inside and outside cells, mutations, a loss of certain crucial cells and an oversupply of others, and the progressive cross-linking of molecules in connective tissue that leads to wrinkled skin, stiff tissues and organ damage. Fix those problems, he argues, and a human being could live to be 1,000.

Sound far-fetched? I agree – yet discoveries in the last 20 years suggest that some of these processes can be postponed.

Stipp does a better job than Weiner of explaining this progress and conveying the mounting excitement of scientists in the field. His central character is David Sinclair, a brash Harvard researcher whose 2006 study of resveratrol’s life-extending effects in mice ignited the interest of investors, drug companies and the general public.

Stipp, a former reporter for Fortune and the Wall Street Journal, also interviewed other scientists in the forefront of the search for compounds that, like resveratrol, appear to activate genes involved in animals’ response to environmental stress. Some of these genes were discovered in mutant worms or fruit flies that lived unusually long; others were found by researchers exploring why restricting food intake lengthens life span and conserves vigor in virtually every species that’s been studied.

Calorie restriction itself isn’t practical: It shrinks muscles, causes fatigue and infertility and makes people miserably hungry. But compounds that selectively activate some genes that are activated by calorie restriction may turn out to retard aging without these side effects.

In a study sponsored by the National Institute on Aging, rapamycin increased the maximum life span of mice by around 10 percent; small trials have been conducted in people with heart disease and cancer, but rapamycin’s net effect on human health is unknown.

The last two centuries have already seen a doubling of average life expectancy, with most of the progress in the past 50 years coming from improvements in the lives of older adults. “The number of centenarians on the planet has more or less doubled with every decade since 1960,” Weiner notes. Longer life spans in developed countries are strongly associated with lower birthrates, leading to changes in population makeup and raising questions about the effect on economies.

While Stipp suggests that anti-aging drugs could deliver a “free lunch,” it seems more likely that there will be costs – for the individual, for society, for the planet. As Weiner writes, “No other scientific program raises so many enormous and imponderable questions, and they are so blithely dismissed by the engineers who would build the dam in the valley of the shadow of death.”