I used Bristol Ground School for self study, but supplemented it with a question bank.
I finished the conversion process about 18 months ago and everything others have said is accurate.
A few other points you should be aware of:
The initial medical exam must be done at the official CAA medical centre(s). Subsequent medicals can be done through private AMEs similar to the FAA. The country in which you do the initial medical exam will be the country which issues your ATPL. The 14 exams can be done in any country, but they must be all done in one country. In other words you cannot do 7 in the U.K. and 7 in Ireland. But you can officially transfer the results from one country, to the country which holds you medical records. I did the exams through the U.K. but medical and license issue through Denmark.
Officially any EASA approved examiner can do your ATPL skills test, but if they are certified through a different country than the one that will issue your license, there must be a pre-approval of the examiner. The pre-approval usually only takes a few days, and it is really just a matter of the examiner registering on the other nations CAA site to view any differences in requirements and get a copy of the paperwork. Now be aware that "officially" does not mean it will always be smooth. The southern European countries, Spain in particular, are notoriously difficult to work with and getting approval for a TRE from another country can be at times impossible.
The last big thing you need to be aware of is that although it may not always be listed, many CAAs require a radiotelephony license as well. And no, the FCC license will not work. This often requires a separate radio theory exam on top of the 14 ATPL exams. The U.K. is one of the countries which have this requirement. Ireland does not have the radio license requirement.
The U.K. has the most experience and best defined process for for the conversion, however they can be a bit more on the expensive side. Ireland has a very good reputation for being easy to work with. I have heard the Icelandic CAA is also very easy to work with. Denmark is also easy, but expensive. Again, the southern European countries can be cheap, but notoriously difficult.
Oh, and if you go through Germany, be aware that German privacy laws prohibit the sharing of medical information. What that means is if you have a German license and need to change your state of license issue to another country, it will be difficult. Any EASA license can operate any EASA registry, so in most cases it will not matter. But there are a few airlines which require you to change your license over to their country to make the LPC paperwork easier. EasyJet is one such airline.
Feel free to ask if you have more questions.