They're as reliable as whoever posts them.
Books will be more reliable - simply because they have to go through a professional editing and publishing process, with (in theory) some degree of quality control.
Anyone can post net tab - no quality control whatsoever. (Other than other members of the tabbing community - if there is such a thing - posting corrections. If net tab operated like wikipedia, it might be better. Still not 100% of course.... ;-) )
Treat any tab as a guideline only. It's what someone else thinks was played (and how it was played) not what the original player says. Always check with the recording and keep your ears open. If it sounds wrong, it probably is wrong.
(But also make sure you understand the tab correctly of course, and bear in mind that it can't show every possible sound accurately. Even the best tab is going to be incomplete or crude at certain points.)
Ultimately, you have your own ears. I always transcribe songs myself, using net tab only for a second opinion. Sometimes (not often) it's better than me! But it NEVER gives me everything I want.
Transcribe yourself, and you will get a greater sense of understanding of a song.
Here's some software to help: