Al-Waqidi is reliable for purely historical reports. Ahl al- Hadith consider him too honest and too rich a source to be discarded especially in light of Ibn Sa`d's accreditation, which lent him huge credit--but they unanimously discard him with regard to ahkam reports which are uncorroborated by other narrators e.g. wiggling the index finger in Salat. It is the latter category they meant when they called him a liar, i.e. thoroughly unreliable and/or inaccurate in his isnads, not at all that he was dishonest. Al-Dhahabi said: "I have no doubt in his sidq." And Allah knows best.
Sira Halabiyya; Sirat al-Dahlan; al-Sira al-Shamiyya (the most comprehensive hadith-based Sira); most recently the massive study by Dr. M. Abu Shuhba, al-Sira al-Nabawiyya. Also the Sira by the late Egyptian Shaykh, Muhammad Sadiq `Arjun. A reliable brief Sira used as a textbook in many schools is Nur al-Yaqin fi Sirat Sayyid al-Mursalin by Shaykh Muhammad al-Khudari Bayk.
Important Sira studies: al-Kattani's Ahkam al-Idariyya and the Fiqh al-Sira both by Dr. Sa`id al-Buti [available in English at Dar al-Fikr] and Dr. Muhammad al-Ghazzali.