Watch out!
crazyscientist, I like the fact that you are seeking information!
However--- even though you have probably heard this before--- I must caution you to be very very careful about "free books", particularly ones which have not been referreed, as textbooks published by reputable publishers generally are (for example, I happen to know that many academic publishers referee advanced textbook manuscripts submitted by prospective authors, even in the case when the author is a highly respected expert, and this is indeed useful and appropriate since even experts can make a goof which could damage the progress of students too inexperienced to recognize a problem).
In particular, without naming names, I have grave reservations about at least one of the websites which someone has already mentioned above. Unfortunately, the author of this website is rather notorious for self-promotion of his highly idiosyncratic website, which I would consider to be seriously misleading with sufficient frequency to mitigate its possible usefulness.
On the other hand, I do know of some excellent course notes on various specialized topics which were later published as now standard textbooks, and these would be reliable sources of information. However, in general, if you want to learn a subject strongly enough to be willing to put in the effort required to master a subject in physics, you should probably be willing to purchase one of the best standard textbooks available, and use that as your main resource. Then you can use on-line resources as secondary sources, with due caution if you are not sure you have good reason to trust them.
Many years ago, while I was still a graduate student, at the dawn of the Age of the World Wide Web, I put together some websites which were aimed specifically at helping students find reliable sources of information on some specific subjects in math/science. In those days, the web was young, and many of us had great hopes for how it would develop in terms of making good information freely available to anyone anywhere at any time. Unfortunately, to say the least, things have not turned out as we hoped! Today I would not attempt to put up such a website, because I feel that as things have turned out, there is almost no reliable information available on the web, compared to the overwhelming quantity of well intended misinformation, outright crankery, and worse.
Let me "pre-emptively" add one additional specific caution: be aware that while it is apparently now common practice by students, teachers, journalists, jurists, voters, and policymakers to pretend that the Wikipedia is a stable and reliable source of information, this is not at all true.
I know of hundreds of physics related articles in the Wikipedia which contain serious misstatements, even articles written entirely by cranks which have remained essentially untouched to date by the alleged "army of watchful eyes". To be sure, I could also list versions of WP articles which were thoughtfully and well written by editors who possesses a thorough knowledge of the subject at hand, but even here there is the problem that an article which is good today be much worse tommorrow.
Wikipedia does often offer good information--- the trouble is that it can be almost impossible for a non-expert to reliably tell the good (versions of specific) articles from the bad ones! In particular, be aware that I have seen some articles which would fool most intelligent readers who were not experts, but which were nonetheless a farrago of misinformation, so you really need to be very cautious in using Wikipedia.
I should probably add that while I feel that Physics Forums is currently moderated rather well (in comparison to some similar sites), I have spotted quite a bit of misinformation here, only a tiny percentage of which I have attempted to correct.
thanks a lot for ur replies and for the time that u have spend to write them! well its my first year in a greek university where i study physics
