Alberts is a good read for undergraduates who have taken at least one biology course already (some use an abridged version of the book for intro biology courses). As for studying biology at a molecular level, molecular biology and biochemistry are two topic I'd look into. These focus on understanding the molecules that carry out all of the cellular functions and how those molecules work. Alberts covers molecular biology pretty well, and I'd suggest Lehninger for an intermediate undergraduate level text on biochemistry (I use Voet & Voet as my biochemistry reference, though it's probably geared more toward advanced undergrads/graduate students).