Go through attached book.

Each absorbance peak corresponds to a specific chemical bond, also influenced by neighbouring chemical bonds (atoms bond together). You should read through some literature about this. Some practical details about the ATR-FTIR technique may be found in the following article (which may be requested through Research Gate): B. P. Jelle, T.-N. Nilsen, P. J. Hovde and A. Gustavsen, ”Accelerated Climate Aging of Building Materials and their Characterization by Fourier Transform Infrared Radiation Analysis”, Journal of Building Physics, 36, 99-112, 2012. When interpreting ATR-FTIR spectra where quantitative results are important, note that non-corrected ATR spectra have much stronger absorbance bands at longer wavelengths (smaller wave numbers) than at shorter wavelengths (larger wave numbers) compared to normal FTIR transmittance spectra. The reason being that the penetration depth is dependent on (in addition to refractive indices of sample and ATR crystal, and angle of incident radiation) the radiation wavelength, and increases with increasing wavelength (decreasing wave number).