Are old books useless?

Are old translations and studies which are in bad need of improvement misleading and to be avoided altogether? Does it still make sense to have a first look at, e.g., G. Jhā's translations, or should one rather save one's time and dive into the Sanskrit text, instead of wasting time into trying to understand what Jhā might have meant (given that he could have been wrong)?

The answer is: it depends on who you are. Personally, I always recommend to myself to read more and I never regret it. Even older studies (in the field of Mīmāṃsā, for instance, Georges Thibaut's, Franklyn Edgerton's and Gaṅganātha Jhā's ones) are usually helpful for me. I tend to be better at improving an existing understanding than at building a new one altogether. But other people need to have a "clean" desktop before starting their work. If you cannot start studying unless your flat is tidy, you might prefer not to read something whose theses you would later need to adjust. (If you think you do not know yourself well enough, just focus on the way you work or research every day, look at your desk and at the room behind it.)

As for my praise of reading, you can read this post and this one (my methodological manifesto).