Cajal wrote the first version of the present book in 1897 --- the year another of the great neuroscientists of all time, Charles Sherrington, gave the gaps their name: ``synapses.'' The young investigator he had in mind was a male, Spanish biologist, very definitely an experimentalist (theorists are said to suffer from a ``disease of the will''), without any real financial support for his research. His advice was, in sum: master a technique, do cheap experiments, produce data, let nothing discourage you or keep you from the lab, and work like a demon, for your own reputation and, of course, for Spain's.
Need I say that the book's age shows? The chapter on the ``diseases of the will'' which prevent actual research is still entertaining, and probably every scientist could name at least one case of each from their acquaintance. (Cajal omitted, for obvious reasons, computer maniacs and administrative empire-builders.) The remarks on scientific method are mostly sound but commonplace. One needn't know English, French, German and Italian (!) to keep up with the literature any more; even the French have had to accept the almost-exclusive use of English. (Anyone who can handle this translation has more than enough command of English to deal with the journals.) The long discussion (ch. 6) of what to look for in a wife is now extremely amusing, which Cajal would almost certainly have found distressing. (He commends women scientists as wives, but has no advice for them --- other than to assist their husbands' researches!) So, in a different way, is the idea that biologists could afford to subscribe to even the essential journals, never mind buy apparatus and supplies, out of their salaries. (A biologist friend of mine had some choice, if unprintable, words when I read her those bits.) There are some unfortunate obiter dicta on the inadequacy of natural selection and the ultimate unfathomableness of physiological processes.
Those searching for useful advice for young scientists would do better to look elsewhere (e.g., Medawar's Advice to a Young Scientist); Cajal's recommendation is, in essence, to become Cajal --- a method which had at least one successful field trial, but probably wasn't generally applicable even then. On the other hand, if one is interested in Cajal, who is surely worthy of attention, or at a shrewd look at scientific life before the great transformation of the last half-century by a major participant, then this book is highly recommended, not least because of its brevity.