Must I be a Maliki or a Shafi'i, and which is the soundest of the madhhabs? And what are the accredited books in comparative fiqh?

Both schools are good, and whichever one you adhere to, it will be good, God willing. However, you must study under a proficient shaykh. Among the easiest and most concise books of comparative fiqh is Bidayat al-Mujtahid by Ibn Rushd, and among the most comprehensive and extensive are Al-Majmu' by Al-Nawawi and Al-Mughni by Ibn Qudamah. Among the useful books on the sayings of the early Muslims are Al-Muhalla by Ibn Hazm and Al-Tamhid by Ibn Abd al-Barr. Among contemporary books that encompass the schools of thought is the Kuwaiti Fiqh Encyclopedia.